I don't buy many games now and it makes buying new games at full price really hard ro swallow on my PS5/Switch.
There are times where I'm more willing to buy a game because of the money I've 'saved by gamepass. Bit of a weird one.
Phil Spencer has said a few times that Gamepass Users buy more games (than non gamepass users) but I always assume those subscribers (especially the earliest) would have bought more anyway. They could be buying less than before subbing and still buy more than non-subscribers.
They have mentioned games that have gone on to sell more because of going on gamepass but I think that's case by case. Often they'll be multiplayer or just games that get more word of mouth from gamepass than the marketing they were previously doing (or not doing in lots of cases), and in some of those cases it's actually that they get more sales on other platforms - think fallguys or rocketleague being on ps+.
I'm sure there are many (most) that get less sales on Xbox as a result. I'd imagine Microsoft are chucking enough money at them to make that worthwhile though as lots of devs keep returning to it.