Say, if you told me, they could have used that spot for K.Rool or Waluigi or whatever, we are not taking into account factors like "Is this character I'd rather have really that prominent so that it would warrant a spot?" I don't doubt that there are Nintendo characters that have followers but some of them may be more niche than we think.
[...]Also, if a character ends up getting considered during development, I don't believe that Sakurai says "Yeah, I'm gonna take the assets of this fighter and transfer them into someone they don't expect, just to piss people off". And it's not like he nevers listens to requests either; Megaman and Little Mac happened because people wanted them.
No, it's probably more like this:
Sakurai: Let's make three versions of Mii Fighter!
Later...
Sakurai: Hmmm... who else can we add? Oh we could add Palutena and K Rool... but we don't really have time for both of them... I guess I'll just add Palutena.
(Insert whichever characters you like.)
If not for the three Mii characters, when considering other characters he would consider the pre-existing workload to be lower, and therefore be more willing to add other characters. It's not that complicated an idea, and it doesn't require Sakurai to have any nefarious thoughts or be rejecting a character because he's directly thinking something like "Oh, K Rool would be nice, but I'd rather please casual gamers instead of K Rool fans". It's a simple factor of there being limited development time.
If you think that Sakurai has made any decisions about the roster which take into account development time, then yes, one character can displace another. In all the previous Smash games, the roster size was limited by development time (and we know of specific characters that were not included because of this). So it's not crazy to think he has some limitations now.
I agree for characters like Waluigi that he probably wouldn't have gotten in no matter how long Sakurai was taking... He's simply not high up enough on Sakurai's list - the roster would have to be huge.
I don't think it's a stretch for fans of Isaac, Ridley, K Rool, Dixie, Bowser Jr, or others with much stronger arguments for inclusion to blame him for picking Mii.
They might be more niche than Miis... but that's not the point. Miis didn't get just one move set. They got three. And there are diminishing returns on adding more Mii movesets. K Rool doesn't have to be less niche than Mii for this argument to make sense, he just has to create more hype than adding a
third Mii moveset.
And I think on that basis, it becomes a lot harder to dismiss those characters as "just not being worthy in the first place" which is essentially what you're saying.