No, they would never make games only available in the subscription services. The Xbox execs have been talking about choice and options which is why they have allowed older disks be playable, why they put their PC games on Steam as well as their store and why every game is on Game Pass day one. Putting the games in the service only is not pro consumer and will only lead to long term losses with no real benefits
Microsoft loves subscriptions - Office 365 and Azure are taking over Microsoft's business revenue. Long term, gaining a huge base of low-spend subscribers and nearly guaranteed yearly revenue is way more valuable than a small number of big spenders. Consistency and predictability are really valuable to a large business like Microsoft.
I'm buying a PC soon anyway, but if I wasn't then deciding to lock exclusive games on Gamepass would royally piss me off to the extent that I wouldn't buy the XSX.
Call me old fashioned, but I like options. I don't play that many games in my old age, so Gamepass isn't suited to me at all. I want to be able to purchase what I want, when I want and how I want. Could you imagine if your whole game library was just locked to Gamepass? What if you experienced financial difficulties and needed to cut the sub? Bye bye games.
Nah. Let me purchase my game as a digital download or a disc.
It would make sense if there was a ton of competition, but as of now, who is GP competing against? Stadia? GF Now? PS Now? It needs to take off first and foremost, make its baby steps, the plan is set up, now MS needs to execute it and see how it plays out, but I honestly don't think MS decided to open up on as many platforms as possible, only to close some of the games behind GP wall.