Unless it gets a PS5 & Switch 2 version, I don't see Starfield pulling more than 14 million in lifetime sales. At least 60% of those will be on PC, and then there is Game Pass on both Xbox and PC acting as an alternative which will drive down the need to buy the game itself.
I think it can do a solid 3 million in its first month (1 million on launch day), 7 million in its first year, and then sell the rest over a period of a few years. Again, that's with Xbox & PC, majority of sales on the latter.
They aren't getting Skyrim 30+ million numbers without PlayStation and Nintendo, full stop. You can kill that dream right here and now. Skyrim also benefited from a very different market at the time where there weren't many other WRPGs around, and RPG mechanics weren't as prevalent in other AAA games. That's no longer the case, and it will still have to contend not just with Spiderman 2, but also Baulder's Gate 3, Diablo 4 (which is still going strong), TOTK (also still going strong), and FF XVI (ditto). People aren't just going to stop playing those games because Starfield's now available, no en masse.