I think we can start to put a good perspective on what GamePass truly is: a litmus test for game quality justifying (or not justifying) a sale. Which also means it as a curation tool naturally enforces a weeding out of weaker content, because when the content is "free", people are going to value the content of higher quality to make use of their time, which is in some cases an even more limited resource than money.
There has to be a very interesting psychological effect at play here, too. As in, if a product is good enough, there are still a sizable number of people who'd want to buy it even if it's "free" in a service, just to have that sense of individual ownership and investment into something deemed of quality. It's kind of like while at a restaurant and you tip the server; you don't HAVE to do it, but if their service is exceptional, you do so anyway not just out of generosity but because you want to reward them for putting in great service. I think there's a similar mentality among a lot of gamers when it comes to buying games like FH5 even when they are providing themselves "freely" in GamePass.
Conversely, if a game has a lot of flaws, bugs, and just generally turns out to be unpleasant, then that game is likely to see a decrease in sales. The important thing to remember here is that said decrease would come due to the game's own lack of merit, in whatever areas it's lacking, again going back to the server and tipping example where if you have a server providing poor service, you'll probably decide they aren't worth tipping. Outriders is a good example of this, but at the time news came out of the devs missing royalties, likely due to softened sales vs. the forecast, I didn't see many people pin that on the game itself having a myriad of self-inflicted issues. In fact a lot of people blamed GamePass suppressing its sales, but if so then GamePass was in fact doing its job: amplifying quality releases, and exposing weaker ones (we're not gonna get into Square-Enix's questionable practices as a publisher here).
Some people think GamePass is meant to be an equalizer, but they may not know what that truly means. IMO, it means that services like GP remove the blanket of marketing glitz for otherwise mediocre games that would get a pass into tricking a customer of purchase through the traditional model, because they could fool that person with slick ads, or paid reviews, obfuscating the customer from playing the game for themselves before committing to a purchase. When a game's on a service like GamePass, those type of tricks don't work as well anymore; the game has to stand on its own quality and merits to convince the user it's something they want to invest further into, whether that be time played, additional content purchased, buying the game outright, or a mixture of all three of those things.
It equalizes the field of presentation to the would-be customer, giving them an honest means of trying before buying, and the aforementioned psychological element akin to tipping in the services industries, likely being at play subconsciously among customers within the hobby of gaming as well.