For GAAS, it's simple.
Sea of Thieves and Grounded have been big successes, yes, but they've been exclusive to Xbox for years, the Xbox hardware has a relatively low ceiling, Game Pass likely has a strong attach rate for Xbox, I bet most who are interested in Sea of Thieves in the Xbox ecosystem have already played it. It will one day reach a saturation point locked to one piece of hardware.
The biggest GAAS in the industry are on everything, Fortnite, Call of Duty, GTA V, Minecraft, Roblox, etc.
At some point, a years old game will hit a ceiling on the players it can reach, and then the question becomes simple, do you expand its reach to more players by porting it elsewhere, which can lead to further investment and a large injection of players or do you shut it down and move on, the answer is simple if you care about the IP and want to continue to see it thrive with an active community.
That's not to say Sea of Thieves is on deaths doors, it still has a very active community, but it could be a lot better, a lot bigger, if it doesn't restrict itself, it has almost certainly used up most of its console selling power without doing a Sea of Thieves 2, it is 6 years old, so you're keeping it exclusive to Xbox for nothing while porting it everywhere COULD help elevate it into the next big GAAS.
It's already 30m between Xbox/PC.
At some point, it becomes dumb to keep GAAS exclusive to a single platform.
Bungie knows that, why Marathon is fully multiplatform 
