Gabe Newell believes brain interfaces will create games 'superior' to reality 'fairly quickly'

It sold out immediately when it launched, and when stores got new stock

That's the oldest marketing strategy in the book though. "Oh no it's all gone already! You'll have to wait for yours while we make new ones" "But now I waaaaaaaaaaaants iiiiiiiiiit!!"

Besides, when is something "mainstream" then? Seems pretty arbitrary

It is. It's usually defined by a ratio though. There are literally billions of gamers worldwide. Millions and millions not being a very clear figure, it's hard to define a ratio

I'll give you 26 million units worldwide (privately owned)
Rough estimates of the gaming population count people who own gaming-only devices (like consoles) as well as people who play once a week or more on their PC and does not count Android gamers for example and the estimate is 2.7 billion gamers.
That means VR headset owners account for less than 1% of the gaming population (0.96% to be exact) , and that's not discounting the fact that most people who own headsets probably own more than one because they love shiny new things and can afford it so they're more than likely to buy new models when they are released.
So mainstream, no. Growing? sure. At a fast rate? Yes. Does that mean it's the future? In its current state, no. Even if they did sell another 10 million in the next 2 years, that would still remain marginal.
It sure does sell companies on the stock market though, hence the "future is now" announcements like Gabe's, based in zero reality but innovation sells stocks.

All the stats come from Statista, an independent source, which I used mostly because most other VR stats I see seem to come from sellers or people trying to sell VR to the stock people so it's not very reliable and I thought having both stats from the same source was only fair.