Making TES6 gamepass only would only result in a massive gamepass subscription boost, which is what MS wants. If there's one thing 40 years of home gaming history shows it's that people will do what they have to in order to play the games they really want to play.
Generally that's true. Steam was launched with the hype for a single game. Not sure how well it's working out for Epic though.
The thing here though is people like to own their games. They like a back library of titles they can thumb through and play whenever they want. Gamefly is still around, costs a reasonable price, and lets you play any game you want. Why hasn't it completely killed the rest of the market?
Right now Gamepass seems like a good deal. When cable TV first came out it was a pretty good deal. Lots of content, no commercials, low price. Now look at it. Pay through the nose, commercials everywhere, content is all crap, everything good costs extra. If a new hot movie was released on cable only, would you sign up just for that?
Look at Disney+. All the great disney content. Beautiful. Then Mulan comes out. $30, plus subscription. GTFO of here. A bluray costs less than that. Do you think Mulan added subs or cancelled them? I wonder.
Look at Kindle Unlimited. Great value, if you go by number of books. But everything there is crap. All of it. No good books come on on KU, because any good author can charge a premium. The content is commoditized, the profits all go to Amazon.
If TES 6 comes out on Gamepass, will MS give it away to existing subs? Lol. They already have that money, something has to pay for that 7.5 billion dollars. Expect some kind of 'gamepass premium' where people have to pay extra to get the game, just like Mulan. So the benefit of gamepass is nothing; you're just paying an extra fee to access your own library of games. That's why all subscription services are doomed to either become dumping grounds for trash like KU (and soon to be Netflix) or a nightmare of gouging and tiered content like Cable.