My next question,
Why keep Anthony, but not Adam or MB?
Much like Sylux and Weavel, Anthony has a history with Samus (and a positive one at that). Metroid always has been that franchise that seemed like it never had a logical second character next to Samus (these days, everybody advocates for Ridley because he does make sense in terms of importance).
But let's say he couldn't make Ridley fit or decided that Ridley should remain a boss. He should be aware that a large variety of fans are desiring the Metroid franchise to grow. So who are his options?
Dark Samus; The main antagonist from the more popular games in the early years of the Wii's lifespan (Follows the trend of having more Samus' being playable).
Anthony; a colleague of Samus's from Other M, the game the Metroid franchise and Samus are more based off of (Who is not Samus in any way, shape, or form. Which could be what Sakurai thinks we want if not Ridley).
Both do make sense in their own respects, not more than Ridley, but if the situation ends up not having Ridley be in the game, those two are the next best options.
Adam is dead, so in canon, it wouldn't make much sense (But then again, canon doesn't matter in Smash. So I guess keep him too then).
There is no real interest in adding MB other than the fact that she is MB in human form. Any support for her seems literally non-existant.