His goal is artificial general intelligence, something that seemed to grab his attention after he was already actively working on VR. My guess is that when he started teaching himself machine learning, he realized that he needed to focus all his attention on it if he was going to take it seriously. Facebook's failings just accelerated this for him.

I think the truth is that VR is there. They've sold lots of headsets and people who love it, really love it. Expecting it to sell at iPhone levels was never going to happen.

So by "there" you mean that the tech is good enough to provide a good experience, and so they've basically plateaued? That's my guess too. I rented a Quest for a couple of weeks and it was pretty neat. It just didn't fill any needs we actually had. I was thinking of it as a try-before-I-buy situation, but when I sent it back at the end of the two weeks, nobody cared. The kids were already back on their Switches and the Playstation for gaming.

Yeah, that's pretty much what I mean. They aren't some new thing that either has to launch into orbit or crash and burn. The likely path is in between.

VR headsets have improved immensely since I first tried it in 1991 (Dactyl Nightmare) and I suspect they will keep improving. Where we are at today is not necessarily a plateau but it might feel like one if you were expecting a growth curve that looks like a hockey stick.