This is the problem, you are not paying for the same thing. Also, MS would not make more money with it if it was a better deal for you with streaming.
You can see them moving away from the day and date already with sneaky paid "pre release" access to their games, putting a lot of emphasis on Indie Games, sometimes old ones at that (I love indie games, but they're not the same thing as AAA, better or worse is another discussion).
Just there the cost benefit changes a lot, it's no longer about being able to play a decent number of tent pole titles on release with access to the other games as fillers during the down periods. It's about having a number of cheapo games on rotation with some relevant AAA games in release from time to time.
When I buy a game, on any platform, I get something because I think that I will care for it, enjoy it while I play, I don't care about the other games at that time.... If I paid a monthly fee most of the money would go to waste, simply because I buy a few games that I positively know will be meaningful to me and I spent significant time on them .
How many games will you play in a year? After two years how much money spent on GP + games you had to buy anyway? Put that over 5 years and tell me who gives you the best deal, assume you buy most games on sales with a few day one because sometimes you get on the hype train.
Pre-release? You mean the four days early access for Forza Horizon 5 which im pretty sure was the only game that Microsoft has done this with for this generation and even then, it's the higher editions of the game which you don't get in Game Pass anyway. You only get the base edition in Game Pass. If you're someone who's going to play Forza Horizon 5 daily or even weekly and stay with it for months/years, it does make much more sense for that individual to buy the more expensive edition to get all the extra content as well as four days early access. Microsoft goes after Indies because they're cheaper but also at the moment, way more of them than AAA or even AA titles on a weekly/monthly basis especially over the last few years.
Cost benefit will change a lot based on what's actual releasing. If there's more Indies releasing than say AAA/AA games, not much they can do about it.
When I buy a game, it's because I want to play it and because for me, it's worth playing. I don't pay a fee every month. I only pay when there's a game I want to play that's available day one and once that game is completed, I cancel but the vast majority of those who pay for subscriptions aren't me and don't do what I do. They stay subscribed. I'm in the vast minority. But I do get what you mean though.
The amount of games I play in a year is all based on if the games I want to play get released. I had 5 games for XSX/PS5 launch in 2020, 10 games for 2021 and I have 10 games for 2022. For 2023 though, im expecting an easy 20+ games year especially since a lot of 2022 games were delayed into 2023.
Your scenario of 2 years of Game Pass plus games I bought anyway doesn't apply to me though as like I said earlier, I only subscribe for the month. The best deal FOR ME is Game Pass due to how I use it to my benefit. I do buy nearly every game that I want to play day one. I rarely wait simply because im someone that has no interest in having a backlog due to the fact that in past generations, if I ended up not playing a game, it simply gets dropped/eliminated and I move on. I do like to play games in their respective release year and rarely go back to previous years.
Thus far, I have saved $100 on Gears Tactics and Halo Infinite. This will increase to $150 once I play and complete A Plague Tale Requiem. As of now, I have at least 4 games for 2023 that I want to play which will be on Game Pass day one which will save me at least $200. This does NOT include Microsoft's first party exclusives like Redfall and Starfield which increases my savings.
Of course, im not someone who stays subscribed to subscriptions if im not using them on a daily basis and unlike most average people who have kids and far more expenses than I do, it's actually tough to use me as an example because I don't do what an average person would do in this situation where they would just stay subscribed and that's it. I'm simply in the vast minority in this regard because of how I buy/play games and how I use Game Pass to my benefit.