I think people are too cavalier thinking anything is possible for Smash. I think a good good deal is within Smash's reach, but 'anything' is a bit hyperbolic.
Last time Sakurai was not able to acquire Snake. We didn't find out about that until this time, when Snake came back. Had Snake never come back, we probably would never have found out that Sakurai had attempted to re-include him. We know that prior to saying yes on Sonic, Sega said no, then came around. Had Sonic not been included, it's possible that information never would've come to light.
We only hear about the failed attempts and the close calls when that character actually makes it in. Those that don't make it, we won't hear about. So I think it's a bit naive to think that over the years the successful acquisition rate has otherwise been a solid 100%.
I know some people think it's crazy to suggest a company would either not jump at the chance to be included, or would fail to strike an agreement with Nintendo, but I think it's crazier to think that there has never been a third-party that failed see eye-to-eye with Nintendo on this before.
It's not that uncommon for a company to have weird hangups or restrictions on their characters, especially their popular ones.
This isn't specifically about Microsoft, btw. Microsoft seems willing to play ball. It's just in general.