Yeah, no one's leaving Nintendo for Sonic if MS decided to make Sonic games exclusive. I also think in that case, they'd face a much nastier Nintendo because Sega is Nintendo's biggest 3P supporter AFAIK.
I would say by virtue of sheer amount of IP Sega have, even if some haven't aged particularly well (in some cases because their genres are dead), they still have enough to where a creative platform holder could do some really neat things with, like Panzer Dragoon or Dragon Force.
Problem is, Microsoft are by far the least creative platform holder probably in the industry's history, when it comes to actual game content. This isn't the same thing as saying they don't make quality games: they do. However, when you look at Microsoft's ability to leverage and maximize their IP for long-term value retention (both in terms of revenue growth and expanding the fanbase, as well as quality control), they are maybe tied with Sega at historically being the worst at it, although I think with Yakuza and (though via acquisition) Persona, Sega have been better at it now than Microsoft.
So when I look at Microsoft's IP and see the declines in things like Halo, the stagnation in things like Gears and Forza, the difficulty they've had introducing a good number of new big IP over the past several years with clear staying power, the lack of being innovative with use of IP that could've kept their brand strong (Banjo, Perfect Dark, Crimson Skies etc.)...I doubt they would really know what to do with Sega & Atlus IP that aren't already active.
And even with the active ones, what would Microsoft bring to propel them to bigger heights in terms of polish, game design, production values/spectacle etc.? The only good thing Microsoft seem to be is as a bank to fund projects, but having no general input or influence beyond that. So what's the impetus in gaining ownership of these assets outside of just wanting more money?
