You mean aesthetically? A palette swap of Plussle and Minun for Pikachu would look so weird it's not even funny. Also, technical difficulties is a legitimate reason, but that's no reason to exclude them in a later game. And besides, aesthetics don't even matter. Many people liked Dark Pit, Dr. Mario, and Lucina even though they were "the same aesthetically". The only reason they stopped liking them was because they were clones. In my opinion, if you give the character something that is unique, and makes sense for that character to be doing it, then it adds something special to the roster. So even though Plussle and Minun are similar to Pikachu in looks, they could still bring something new.
I respect that, but I think I shall have to reiterate the crux of my point.
My point is not that they're inherently dull, it's that there's no real reason why they should be included just for their own sake. What do Plusle and Minun do that
no other character can possibly do?
Team-up is (or was) done by the Ice Climbers.
Discharge mechanic is (or was) done by Pichu.
Power increased via damage is done by Lucario.
Cute little rodent with puffy cheeks is done by Pikachu.
Hey, if you want to bring back Pichu as a freebie clone and have them as alts, then by all means, go ahead. But I struggle to justify including two Pokémon that are almost entirely palette swaps of an existing character just for the sake of "diversity".
It's not diverse at all, it's just including two
potential characters from a previous game for the sake of including them. And that's why I can't support them.