It's so sustainable they already increased PC Gamepass monthly price recently from $1 to around $4 or $5 in Europe iirc.
The gamepass model is hardly compatible with big AAA studios and high budget AAA games, that's the problem. Some of the studios they now have like Bethesda (if it goes through) will demand high costs including wages every month. It's pretty hard to make it in the green when you give away $70 games for free day one in the pass. Either the price goes up like Netflix or Spotify did, or the quality and daily costs must go down. Or you have to sell biggest games on other platforms too.
The other problem I see is we're more time constrained in gaming than money constrained. Our gaming time is restricted by our daily lives (job, wife, children...) more than by money. It means a "cheap" option like gamepass isn't that interesting when we could simply buy the best games you want instead (best PS5 exclusives, best PC games...) Why would we choose to play other games in our time than the ones we want to play ? It doesn't cost that much anyway. On the other hand, our time is precious and limited.