The thing that is missing are the huge AAA games, but I honestly can't see most of those coming to Game Pass. They may come but like 1-2 years after initial release.
Minus like the Forza and Halo series, most of the MS owned studios developing games are fairly shorter in length. Don't take that the wrong way though, even though they are shorter some are real gems. Just don't expect to play every game on there for 20+ hours or anything.
Plus, GamePass locks you in like consoles once did. That stickiness in recurring revenue is hugely valuable in itself, and because it has a network effect (this was basically the reason for the ongoing console wars).
Combine that with cloud gaming, and the lock-in and UX just gets better.
Their main barrier is good content. Thus the splashy buy.