Are titles devalued to zero revalued after some time?

Nope. Once there is a mass giveaway of more than 10,000(?) copies, I believe, the CV is set to 0. It remains there.

I guess it's only fair since it prevents abuse. There are people who would stockpile these freebies just to get easy CV. It's unfortunate for those of us who maybe get them in bundles later and give them away, but... can't complain really.

It has happened a couple times in the past with games that had a very limited number of free copies given, but it is very rare. For the most part, any game that is given away for free will stay on the 0 CV list forever.

DARK SOULS: Prepare To Die Edition was one of the games.

What the others said.
As a matter of interest, what was the game(s) you were referring to?

Thanks, @ShannonA81 and @AllTracTurbo. Yes, it's unfortunate for latecomers. But it is what it is.

As I recall:
In the circumstance that a developer expires any remaining free promotion keys, an end date is added to the game in SG's free game listing, allowing copies given away after that point to once again give CV. Otherwise, since the keys don't expire, there's just too much room for exploiters (of which SG has some self-admitted, mindblowingly opportunistic ones) to simply hang on to keys until their CV recovers.

If I'm not mistaken this can also happen with games that have been removed from the Steam store since the point value can become 0.