Take Elder Scrolls VI. Microsoft would lose a ton of sales on its own platform if it was day one on Game Pass, and who knows how much that might increase existing subscriptions, given that most Xbox players are either in or out already. So, you might need to give it to Sony where it would probably sell tens of millions of copies, and you would reap at least a lot of that as the first-party publisher. But then your coveted big release is practically a flagship PlayStation title, given its install base.
However, if you take Elder Scrolls VI out of Game Pass, you not only get 100% platform sales instead of a smaller fraction due to the day-one philosophy, but that could be enough to play keep-away from PlayStation and be the only console to have an enormous release like that. That is something that could move consoles in a significant way. Add Fallout, Halo and other titles, and you can see how this could reposition Xbox’s entire place in the market. I don’t think it needs to be every single game, but big ones? Some, at least.