Just to give more context to what typical FF games actually sell lifetime-wise (
source):
FF VIII: 12 million (863K PS1 demo, 8.6 million PS1, 1.8 million Steam, 1.4 million PS4, 111K XB1, 29K Switch)
FF IX: 8.9 million (5.5 million PS1, 1.7 million PS4, 1.1 million Steam, 172K Xbox)
FF X: 8.956 million (PS2)(*not counting reissues or the spinoffs)
FF XIII: 9.6 million (5.275 million PS, 2.33 million Xbox, 2 million Steam)
FF XV: 10.27 million (5.7 million PS, 2.3 million Xbox, 2.5 million PC/Steam)
That's a lifetime average of just under 10 million for each mainline FF game outside of FF VII and earlier. I think some people feel these games have always done Elden Ring or BOTW, or Spiderman & modern GOW numbers; that just simply isn't true and has never been true. However, historically Final Fantasy has been the most prestigious of JRPGs in terms of genre innovations and sales.
Historically.
JRPGs aren't even so easily defined these days and a game like Elden Ring could be called a JRPG or WRPG very easily. While we could look at these numbers and see that Xbox didn't do terribly for XIII and XV (at least compared to Steam/PC), consider that FF VII Remake has sold around 9.2 million copies, with 8.5 million being on PS4. That's about in line with FFX and, similarly, on just one platform.
Basically, FF doesn't need Xbox in order to reach 10 million sales, which would be the average for a FF game, and considering a lot of Xbox gamers have switched to PC anyway, Steam/EGS etc. should make up for a good portion of Xbox's missing sales once XVI gets ported to PC. Now there could be a question of if profit margins per entry are as good as previous entries, since AAA games cost a lot more to make today than back in 2001. But again, that's probably stuff Sony factored in when they reached whatever deal they seem to have with Square-Enix, so Sony is footing a good deal of the production costs for XVI (as well as a lot of upfront marketing costs) would be my guess.
That should leave Square-Enix with fat profit margins. XVI is probably already turning a profit for them and should continue doing so with long-term sales plus getting ports to PC. What I do kind of expect though, too, is probably a scaled down (visually) Switch 2 port either in 2024 or 2025, depending on when the PC port is done (that one would come first). If Square-Enix are counting on reaching well beyond 10 million lifetime sales, they'll do that with a Switch 2 port.
Which makes expending resources on an Xbox port practically worthless. It is what it is.