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To me, what's killing co-op games is mediocrity.

There are so many games on Steam, and most of them are lackluster.

I don't play them anyway, since I can only play with friends and I don't have a lot of those, and they're also in pretty different timezones with different schedule.
Singleplayer is like your hand. Always there for you. ( ͡° ͜ʖ ͡°)

You can literally finish Borderland 2's story mode in a few hours (which usually takes about 50+ hours) if you have a co-op partner whose level is lvl 40+. All you have to do is keep running towards your objective and let your co-op partner kill every thing in one blow. You keep gaining experience and if you die, your partner can revive you asap. I actually did this when I called my friend for co-op of a few missions whose level was 75. This reason for me was the downside of playing co-op game.

if a game is good, people will stick to it. it needs support (like updates and an anti-cheat system) to succeed, or it will get boring and frustrating.

i think the problem is developers releasing copy-paste games. they are all the same (reminds me of the moba era with 9999 games like dota and LoL).

I don't really know about online co-op since I have no interest in it, but I definetly think there aren't enough local co-op games.

I'm not sure what kind of co-op games you are talking about. Guns of Icarus for example (while being co-op too) is mainly a PvP experience. And yes, there are probably too much PvP games out there. If you are looking for co-op games then there are a lot which are actually some really good games which can be played without issues in single player but make a whole lot of fun playing in co-op. Especially most games with local co-op.
But if you are looking for always only PvP co-op games then another reason for them failing is that they usually attract a very bad kind of "players".

Right, I think you're missing some big things here.

1) Co-op's usually made for friends, so finding people on public co-op is like finding fallen apples after they've been under the snow for a few months. There will be few of them and they'll most likely be rotten. Meaning that you won't have a lot of people regardless and those experiences will suck most of the time anyway because of a lack of communication and a common goal.

2) The high amount isn't killing them, it's instead what people perceive as co-op. You can, of course, have the Magickas', which revolve around actual co-operation, but also heavy trust for the other one to not screw you and not to screw up, which in turn, again, would screw you.
You can also have games like Kane & Lynch 2 and Victor Vran which are essentially just the single-player experiences that tack on a second player as an extra hand and do nothing.
Some have tried to overcome this, like Splinter Cell: Blacklist, which gives co-op players more missions and extra paths to take. But instead of enhancing the game for co-op players, it instead just punishes those that play alone.
And then you have games like Monaco, Overcooked, Portal 2, which actually build themselves around having co-op and working together.

So, I think, the worst that bad co-op games can do is just make some older co-op games harder to find.

I mean it's right there in the name - "cooperation". Online multiplayer gets ruined by there being too many different ones, since it relies on matchmaking and competition. Co-op games on the other hand rely on cooperation, down to and including cooperating to find a good time to play with your friends, and a game you all want to play.

I mean I'm sure there are some people who only play with strangers, but I don't think that's the main target group for co-op games.

I love co op games and wish there were more decent ones around.
I got a few friends I would like to play co op games with but it's hard finding a game we would enjoy.
Portal 2 was great. The Crew was great. So was Sanctum 2. Wildlands, Forza, ibb and obb, Lara croft, orcs must die, borderlands 2, left 4 dead, vermintide, gauntlet were just some of the games we all found boring. These are not necessarily bad games, but they didn't hold our attention.

I'm enjoying fortnite and nfs payback is next on my list to try.

If anything we need more and more games with innovative ideas.

The problem with online co-op (and all online games really) is that they have to really stand out and be better than the other games out there for people to stick around and keep playing them. Yu can have a "pretty good" single player game that's still worth playing, but if an online game is just okay people will leave it for greener pastures. I think it's the medium that sets the standard for these types of games higher than others.

People are always worried about more games of X category... the more the better... just pick the ones that u like, play them and enjoy them :)

What makes co-op games die is not enough attractive content to keep the players rather than over saturation. I have more hours in Payday 2 than Killing Floor 2, despite both games being similar PvE games, is because I can mix and match weapons that I bought and each weapon has their own mods to further diversify their uses. Killing Floor 2 weapons are bought in steps. I kill zeds with my starter pistol to gain cash for low priced weapons, then buy mid tier weapons from earning cash by using low tier weapons, etc. I have to restart the weapon buying process for each game. This weapon system was one reason I stopped short on Killing Floor 2. The other cause is the results of a mission lack any effect(s) afterwords. I at least earn a daily crate per day from playing, but that is all. For Payday 2, I earn money from completing missions - the game is all about cops vs robbers after all - which I can spend to customize weapons, masks, etc.

I moved on to Warframe now and the grind is alright for a free to play game. I am happy that all items can be obtained by playing the game. The maps feel the same to each other, but the parkour movement keeps my attraction. Great to fire a shotgun at a target while I performed an air corkscrew or run against a wall then jab an enemy below him or her.

I do not think co-op games will be a current trend yet when PvP games like PUBG or Fortnite are still the talk in the video game community.

Guns of Icarus is a Team PvP game, not a Co-op game, unless you mean Alliance. Likewise, a lot of what you're discussing seems more aimed toward the competitive scene (as usually, co-op is something you aren't as reliant on a general online population for [co-op based MMOs aside], with players normally relying instead on friends and other long-term acquaintances for grouping). Substituting "Multiplayer" in your OP in place of any specific sub-genre would presumably much better cover the topic in question.

* PvE / Solo / Single-Player: A single player opposing AI.
* Team PvE / Cooperative / Co-op: A team of players opposing AI.
* PvP / Competitive: Players opposing players.
* Team PvP / Team Competitive: A team of players opposing another player or team of players.

* PvEvP / Competitive Co-op:
A rarely utilized subgenre of co-op in which pvp and co-op is utilized (for example, competition for score in an otherwise Co-op game).
* PvPvE / Environmental PvP:
"Any pvp mode in which there are significant, persistent non-player threats, and defeating other players is not required for progression".
  This sub-genre of PvP expands upon PvP by allowing meaningful interaction with a non-player faction of AI opponents or a persistent environmental challenge as part of the PvP experience. This is in contrast to PvP games where the AI/environmental elements are only there to supplement core PvP, such as by substituting in AI characters for missing players, or interacting with players in a non-coordinated way (eg, environmental hazards).
  Pvp-enabled open world survival games, non-pvp-restricted pve zones in MMOs allowing for progression via either pvp or pve, and games with coordinated neutral AI factions are all examples of this genre. A horror game in which players are running from an AI killer but can kill off other players, with the aim of being the last survivor, is another good example.

As PvE and PvP are the encompassing genre labels (with the team variants as sub-categories thereof), you can categorize games with multiple options under their primary label- ie, PvE for a game with both solo and coop options, and PvP for a game with both individual and team competitive modes. Naturally, clarifying the available play options is preferable when possible.

Further Reading:
https://en.wikipedia.org/wiki/Player_versus_environment
https://en.wikipedia.org/wiki/Player_versus_player
https://en.wikipedia.org/wiki/Cooperative_gameplay
Co-optimus has had a few mistakes in recent years, but it's still the best place to go to get a reliable idea of what the co-op label encompasses.

That's one reason I am coming to really appreciate games that allow using dedicated servers... especially if they also allow for custom settings and/or modding (for example, both Ark: Survival Evolved and Conan Exiles allow this... would love to find more games that support dedicated and that are moddable).

Right now, this seems to primarily be utilized for sandbox games... but I see no (technical) reasons why game developers couldn't use dedicated servers for multiplayer maps or even co-op campaigns. Personally, I would love to have my Linux server run the show and then have me and 1-2 friends play through a campaign running off it.

Realistically though, I think most game studios don't bother because they think most people are lazy and don't want to go through the trouble (let alone the extra cost) of setting up a dedicated server. That may be true in many cases, but I think that viewpoint seriously underplays how much community can have an impact on not just the life of a single game but also the life of the series. And for any game developers listening in, please please understand that this does not mean I want an MMO or "Some Game ONLINE"... because I will straight up not buy that shit. What it means is that I want "Some Great Co-Op Game" that allows me to split out the "server work" of the game and to run it on my own hardware; no more or less.

I got your point after reading everything.
Because for me.. coop-games are mainly a small group of friends playing together
Not an online community
And thats what i guess you wanted to point out. In that case.. yes.. kind of sometimes some games take away a part of the playerbase from another game.

Keep talking and nobody explodes and Duck Game are awesome games for co-op experience

Co-op is nice but bots are underrated and neglected.
I used to play some old squad-based game, probably about 10 year old, kind of like America's Army 2 but with a squad of bots (I forgot the exact name): it was really nice, as the bots were good enough. Meaning making good-enough bots was already possible 10 years ago. Co-op needs good bots to fill the gaps in case players are lacking. I can't believe it's 2017 and yet tons of multiplayer games don't bother to provide bots, let alone decent ones.

I think too many coop games just rely on a single gimmick (move or die,screencheat and similar) which shortens the games replayability and longelivety significantly. Also too many games are just fun if you play with your friends and not with random people, which of course can kill a playerbase really fast.

Not really that many coop games that it is over saturating the market, and the ones that do exist (Payday 2, Borderlands 1,2,TPS, Saint's Row, Dead Rising, killing floor, L4D/Vermintide, Dark Souls) all bring something unique to the table, and arent really killing each other off.

FPS PVP genre tho? Undeniably.

I wish Valve would wake up and create new co-op game, their coop games are amazing, all their games are amazing...dang...but ye they going for that card game :(