I love the idea of kirby being able to use other people's final smashes, but I think it would be a little too much work for one character. They would have to make special "kirbyfied" versions of Wario Man, and Giga Bowser. Not to mention ZSS's and Pokemon trainer's Final smashes seem like they would be awkward and hard to pull off. I don't know, it just seems like all of that work could have been used for creating something else, especially considering that these Kirbified versions aren't going to be seen as often.
However, maybe I'm exaggerating the amount of effort it takes to give kirby all of those final smashes. For most of them, I'm sure the animation could remain, but with Kirby instead of the actual character.