To be fair, it's not like most of the industry is developing their games around subscriptions right now. In fact, even if they were, the games releasing on Gamepass now were already released/in-development years ago. So of course there is going to be little evidence for skeptics, the industry isn't centered around subscriptions yet. Right now most third party developers just used Gamepass as a nice bonus because of Microsoft's pay offs to get games on the service.
It's also not uncommon that first party games get better/less egregious treatment than third party games in terms of consumer value. That's actually the case in most cases, because they want to sell boxes/services. Microsoft has had micro-transactions in their games for a long time, just because nothing has changed doesn't mean it can't possibly change.
Why would reducing quality of the products increase adoption? Keep in mind, the core aspect of a subscription is growing the subscriber base....with a lot of help from world of mouth and customer retention. These aren't binding contracts with cancellation fees like Comcast.
Also 3rd party devs still are free to sell on other platforms. The lump sum for Game Pass most likely will not cover development. It's just one additional low risk revenue stream. I read from a dev in the Phoenix Point forum that the only stipulation they got from Microsoft is that their game launches on Game Pass day 1. He also said that Microsoft was looking for diversity of quality Indies for Game Pass. Outside of that, free to do whatever.
I think what we see now will only improve when the subscription base grows and when there's more competition. Nobody is going to tell their friends they need to pay a monthly fee for shitty quality or shitty monetization.