There's a very long tail of interesting games, 150 games at a time just doesn't cut it. When the urge to replay an old favorite comes along, I'm incredibly uninterested in doing the equivalent of checking Netflix to see if it's still in the library. They'd have to have coverage at Spotify levels to make that start to seem interesting.
I don't have GamePass but do find it intriguing. The value prop is completely on the other end. It's not wondering if you can play (or replay) some older game that you want in particular. It's when a new game comes out or you are in the mood for something you haven't played before, you can go to the page and find something to at least try for zero marginal cost. If you play or are interested in a broad swath of games, eliminating that initial hump of whether you want to invest money into it is a different ball game.
It's really is literally just Netflix of games. Not great at all when you want to watch Movie X, better if you want to just watch some movie, and the only way when you want their in house productions which in theory are striving to be high quality. GamePass isn't to that final level of exclusivity yet, but I wouldn't be surprised if some game goes "Only on GamePass" in the nearish future.
It's also similar to Netflix in that if your usecase was the old "Just streaming The Office only" you could probably just purchase it. A mono game player would definitely be better served just buying the title they want for $60 rather than a monthly fee, but it starts to get more attractive at just a few games per year.
But maybe they'll get there.