It's too bad they won't say how many Game Pass subscribers there are.Phil Spencer actually talked about this a bit at E3 - with both State of Decay 2 and Sea of Thieves, despite both being available on Game Pass, their actual direct sales were quite a bit higher than internal projections. He suspects wider availability of the game means more word of mouth, more "oh my friend's playing that game, I should check it out", etc. which helped get more people to buy them.
Obviously this is just two titles they've done like this so far, and they can't exactly A/B test to prove whether it would have done better with or without being on Game Pass, but he seemed very satisfied with it thus far in terms of not hurting their direct sales.
I think if Game Pass was doing as well as they keep saying it is, we wouldn't have been seeing all those $1 for 3 months for new subscribers or $30 for 6 months deals.