Anything is sustainable if you're fine with just throwing money at it regardless. In it's current form it's not sustainable. Even if all of a sudden everyone started paying the full $10-15 a month do you think Microsoft will keep throwing bags of money at developers forever to keep getting big games like DMC5 on it?
If every one of the 40+ million Xbox owners subbed to Game Pass that would be 400 million a month in revenue, the equivalent of selling almost 7 million copies of a $60 release per month, or selling 80 million $60 games per year. That's not including Xbox Live so let's say everyone subs to Game Pass Ultimate instead, that's 600 million per month/7.2 billion a year in game revenue which would the same as selling 120 million $60 game per year. Not including DLC and MTX.
They could fund a $100M budget game every month and have 300-500 million left over for 3rd party games and profit. Yes, I think Microsoft would be fine with that.