I love how many times you call Dr. Mario literally an alternate costume of Mario. With the word "literally" in italics every time.
If he has unique character traits, then he's not literally an alternate costume. I don't know why this bothers you so much that it's all you talk about. Just don't play as them.
It's redundancy on the roster. If Dark Pit was literally the same character as Pit I'd have the same issue, but he's established in his own franchise to be a different character. In a game all about unique characters, be they from Nintendo or third-party, we don't need to characters, just in different outfits. Way back before I knew anything about Zelda I had the same issue with Toon Link, before reading that besides aesthetically having different models, there are different Links for a larger timeline. If there was a source somewhere that stated that Dr. Mario is a completely different person than regular Mario, then having him on the roster is the same as having Metal Mario or Pink Gold Peach on the roster, difference being I tolerate Dr. Mario in Smash, I just find him unnecessary.
There's also something to be said about the fans of his alternate playstyle, as Sakurai has time and time again reworked characters for Smash Bros. Look at Sheik, who lost one of her special moves from Brawl and Melee, and fans had to readjust to a different playstyle. They managed to do this without making Impa a clone of sheik or doing something like making an OoT Shei and a Twilight Princess Sheik.
I don't know, I know Sakurai wants to pad out the roster but in a 50+ game there's really no reason to do that.
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The whole point is that he doesn't. Not that he can't.
Where is the argument?
1) Dr. Mario can't use the spin moves in any of his games, so what's the issue of him using FLUDD
2) The argument is that they are
literally the same person. In any other game, Mario Kart 8, for examples, these duplicate characters were an issue with people, save for if they are different ages like Baby Mario (can also be applied to Link and Young Link) in melee.