Interesting point, but it wouldn't impact any huge multiplayer games really because most/all are not on game pass or are free to play. Like the paid mp in CoD isn't on GP, and Halo mp will be free to play, etc. Maybe games like Fall Guys would spend a few months on Game Pass but then leave, so there'd still be a reason to buy some multiplayer games.
And even if that happens, I think Microsoft is okay with that. By like 2023, I bet Xbox games will launch exclusively on Game Pass on day 1, and then only become available to purchase for $60 like 3-6 months later. Movies & shows do that by debuting only in theaters or on Netflix/Disney/etc, and then become available to purchase later. I don't think Microsoft really care that much about selling individual games for $60 anymore.