Eh, there's a reason I specifically refrained from bringing up P:M Mewtwo in my last post - its not fair to compare a new game in production to a mod of an already completed game.
The P:M team had more liberties - they focused their efforts toward a specific crowd (the competitive scene), they simplified the stages they didn't use directly, and they didn't need to recreate a ton of assets to get the character working. Sakurai's team would have had to create a new model, on top of building new stages, get that character behaving properly among all those stages, get that character balanced between a much larger roster than P:M's, make sure the character was both approachable from a casual standpoint and deep enough for a professional player to appreciate, all the while taking orders from Nintendo and its shareholders.
In the end, it really comes down to priorities - and when we're talking about a character that's historically proven himself to be difficult to implement, it's no wonder why he keeps getting shunted to the bottom. However, with DLC it's practically inevitable we'll see him - if not with ToS, then perhaps with DLC alongside the release of Pokkén Fighters.
So don't be afraid of any character's chances just because you thought of its chances just early. If the character is in, then the developers have been working on it since a while ago already, that's just how it is.
Whatever new ideas come out now, they will surely end being DLC or something like that in the near future.
Quoted for truth! I hope the people dropping expletives and calling for the man's head realize this when they see that one character they didn't get pop up sooner or later.