Except this is a human being and makes no sense. Junior is a ridiculous character already, him spawning Clown Cars fits his moveset. Mario pulls plenty of crap from hammerspace in his home series too, such as hammers and the like. Splitting up the Ice Climbers not only turns them into a generic character that offers nothing (they had to be simple so the 2 in 1 thing wasn't over complicated) but if you were to have Popo spawn Nana for specials only to say, shoot a second ice block, it would just spit in the face of people who used them to begin with. This isn't just changing up a character's special, or modifying the physics of one of their attacks, this would outright DESTROY how the character was intended to function in the first place. And what would this mean for future installments of Smash? Would the team revert the Ice Climbers back to their Melee/Brawl style if the hardware was able to handle it? Or would you keep the bizarre looking "but hey they're at least still in the game" mentality Smash 4 version.
I may not like cuts, but cutting the Ice Climbers is a far better move than simplifying them or cutting it down to one climber just so that there is still an Ice Climber in the 3DS version.