If either of them were in there killing them would be extremely hard, having to decapiate them would be an extreme chore, and considering the amount of power they actually have the demon slayers would've probably lost, because it took tengen, Tanjiro, Zenitsu and Inosuke to heat Gyutaro and Daki, even if you still had tenegn in there the fact that Daki and Gyutaro could get separated and having them both decapiated at the same time would be extremely hard, and with Hantegu you don't even need at explination as to why he'd be big trouble, 4 different extremely strong clones and then having Zohakuten on top of that would disgustingly strong, and having to fight that whilst chasing and trying to decapiate Hantegu it would be nearly impossible. Gotouge saved themselves from writer hell by killing off upper 6, 5 and 4 because there's no way they could've written their death in the infinity castle without having to pull off some bullshit