I mean...what kind of child roams around the newly monster/crazy people infested world with a baseball bat and nothing but fast food for provisions unsupervised with the blessing of both their parents?
Unrealistic scenarios don't remove character traits.
This logic is a bit disingenuous because I don't think there's anything in-game or in supplementary material for the original Ice Climber game implying that Popo and Nana were meant to be children from the start (or really any specific characteristics for them at all), unlike Ness or Lucas who are clearly established as such in game. What Sakurai did to the Ice Climbers is, as far as I know, his own personal interpretation, and I think it'd be fair to reason that a different director (
or a different direction, we could've had child Pit in Smash as well!) could have done things differently regarding them.
It's kind of like how everyone used to envision the hunting dog from Duck Hunt as a Muttley-esque malicious jerk who relishes in others' mistakes, but Sakurai chose to characterize the dog instead as "cartoon dog who does funny dog things and occasionally laughs once in a while".
I guess at this point I'm not really arguing against Popo and Nana being children specifically in Smash, more that it's the direction Sakurai likely chose for his interpretation of the characters and that it doesn't necessarily need to be the "final say" on how people may see the characters outside of Smash's context (or at least, as long as Nintendo continues to do nothing with the IP in-house). But honestly, between them and the gritty Mach Rider redesign he likely came up with for the character's Melee trophy, I honestly wouldn't mind Sakurai directing a whole game focused on his personal takes on reimagining underused Nintendo characters.
Either that or just giving Ice Climber the Uprising or Bananza treatment.
Anyway, to move towards the current topic, I personally think the biggest obstacle for Paper Mario isn't just being another Mario, but a Mario who's still trying to shake off practically a decade of unenthusiasm towards being in Smash. The mentality for basically two entire games (Smash for and Ultimate) was "I don't want Paper Mario if he's going to play like the modern games", and the TTYD remake may already be too late to sway things right away depending on how long SSB6 has been drafted for.