If MS studios made games that were pushing 10M sales on average and if they had a console that was pushing 80M+ in sales with the associated spike in software sales that comes with that too?
GamePass wouldn't exist, or at the very least it couldn't be a priority for them.
Think about it, why would sony make GoT/TLOU2 available on PS now day one when they know each of those games would sell over 6M copies each at $60.
Because getting 60 bucks once or twice a year is less than getting 10-15 bucks per month every year. If you feel your service offers a heightened quality on such a level that this isn't suitable, you can always hike up the price a couple bucks. I hope you don't really expect me to explain to you in detail the math of why long term subscription models are more attractive to every branch of the entertainment industry than one time purchases here and there.
You're incredibly naive if you think GP is going away if/when MS has a stronger market position, or that Sony isn't going to be making a concerted effort to move in the same direction in the future.