I genuinely don't care if it's "sustainable." I care what's good for me, the consumer.
People claim GP is "bad for game devs," acting as if 3rd parties just wake up one day to find their games were magically added to Game Pass. If it was so bad, they wouldn't continue to put their games on the service.
Atlus put all their Persona games on it, then made Persona 3 Reload a day 1 GP game, then just added Metaphor to it, and now announced Persona 4 Revival will be a day 1 game. That's several years of continuing to do this, so obviously they are benefiting from it.
MS put Call of Duty on it last year, and everyone said it would fail. It didn't, still sold a ton, and now this next CoD was announced for GP as well.
So something is obviously working well enough for them. My only interest is "are the games I want to play available on it?" and "is it worth the cost for ME?"
If either of those are "no," then I won't be using it.