It’s simple logic that if you going to aggressively push a subscription service then using purchases suddenly makes no sense. Engagement and monthly active players is actually a much better way to gauge popularity.
The income is steady and monthly. And optional purchases like cosmetic mtx for say Sea of Thieves is where the long term money comes from when you maintain high engagement and monthly active users.
It's easy to pump up monthly active player numbers when a game is free or tied to a subscription service. Just look at Poke'mon Go, with its 1 billion downloads. Now, does that suddenly mean that Poke'mon Go is the best Poke'mon game? Hell no! In fact, Poke'mon Go is a barebones shell of the original games. You'd have a hard time finding anybody listing Poke'mon Go as their favorite Poke'mon game.
Also, keep in mind that how long people play a game is in no way tied to its greatness. A game can sell 10+ million copies and be just a 10 hour single player game. Using monthly active player lists you'd think a game like that isn't very good. On the flipside a game that is manipulative, grindy, and addictive can have a massive playerbase despite being an awful game. All fun games are addictive, but not all addictive games are fun.
What MS really needs to do is man up, release actual sales numbers, and then project how many sales were lost to Gamepass. That would be more honest than what they are doing now. I get it. Gamepass costs them sales. But we are all smart people here. We can all see that if a game sold 7 million copies, lifetime, while launching day 1 on Gamepass, then that is a really damned good sales figure.