With such staggering sales figures, I can't imagine Microsoft wants to let that go. I wouldn't be surprised if they try to lock this game up or even acquire the developers outright.
It's pretty ironic that they got such a successful exclusive right on the heels of making one of their previously successful exclusives, Hi-Fi Rush, a multiplatform game. What may happen is that they brand it as an Xbox game even if it makes it to other platforms.
Of course, they should clear up any of the potential legal issues...
1: It's not an Xbox exclusive; it's also on PC (Steam Early Access)
2: ALL of the sales have been on Steam via Early Access; zero copies have been sold on Xbox
3: The Palworld devs have already said they're looking to bring the game to other platforms
4: Given the CM's views on mass consolidation, I don't think Palworld devs are down with some extended or permanent exclusivity deal, especially on a console with the smallest install base. No amount of moneyhatting will probably change that.
5: The only way it remains an Xbox console exclusive is if it gets PT'd; if Nintendo hits the game with a lawsuit and it has to be removed from sale, other ports won't come. However, even in this case Xbox owners still lose access because it's a game only available through Game Pass. Microsoft would just ping consoles to connect online, check accessed games, and do an update to remove Palworld copies installed on any systems.