New games cost £59-80 on launch day. 2 of those purchases + a DLC purchase are enough to equal 12 months of a game pass. For consumers, it's good value. Sure, not all games are 10/10 but that's no different to Netflix or Apple TV+ shows.In what way is Game Pass unsustainable? You have huge numbers of people shelling out a decent chunk of cash every month, and probably having a higher annual spend from each of them than they would have made on game purchases, and it also acts as an extended demo service for people who might buy games later.
For game studios, gamepass changes the dynamic a lot. From what I have observed, games that launch on gamepass on day one, generate more comments and content about how the game is to play vs fud from those who are on the offence.
Instead of watching 5-10 YouTube reviews and scrolling through comments on whether to buy the game, you can just download it on gamepass and start playing. I think that multiplayer games benefit most from gamepass. Wider access = more players = greater network effect. Palworld is a great example of that.