I don’t understand why people say this because
- Nintendo has never said that, and
- there’s no “fuss” to it, anyway; it’s literally just an extra button press.
In fact, Mario is arguably the one character who it would be the
least amount of “fuss” for, since he’s already the default character that your cursor starts on; there’s no need to move the stick or anything, so what’s an extra button press? It’s
really not a big deal.
Anyway, the fact of the matter is that, unless they’re going to literally tack on two extra slots to the side or drastically rearrange the entire layout to fit 50 icons (neither of which I think is likely at all), they
have to combine characters to make room now. There are a number of potential options for that, sure, but I really do think that Tanooki Mario and Cat Peach are the most likely candidates for that.
For one, they’re the odd ones out in terms of character variants. All the other Mario series variant characters are together on the third row (the baby and metal characters; Dry Bones isn’t a variant, and Dry Bowser’s relationship to Bowser is…inconsistent), while Tanooki Mario and Cat Peach are up there on the top row by themselves. So for that reason it makes more sense to combine those two with Mario and Peach than the two metal characters (plus Metal Mario already has an alt, so they may not want to add more to that).
Plus, aside from the metal characters, those are the only two characters that are viable to be combined with other characters that would leave two empty slots right next to each other; the Inklings and Villagers are separated by Link, and Bowser Jr. separates Dry Bones and Dry Bowser. Nintendo clearly wants to disrupt the order of the character select screen as little as possible, given how they’ve handled it throughout the history of Mario Kart 8, and combining Tanooki Mario and Cat Peach into Mario’s and Peach’s slots seems to be the best way to go about doing that at this point to make room for Pauline and Peachette.
EDIT: And yeah, what
@MondoMega said as well.