Ha.
It is objectively true that Toad is more important than Dark Pit.
It is ridiculous to suggest that importance is subjective. It isn't, except perhaps in extremely marginal cases.
Some of the newcomers are important. Some aren't. But many that are extremely important were excluded.
Goku is only a troll request in the smash community, most are deadly serious.
Firstly, it isn't a slippery slope, it is
Reductio ad absurdum, a perfectly legitimate form of reasoning.
Your 30 mario and 30 pokemon argument is a nice strawman. If you are going to represent nintendo's history in a rational and consistent way, you would include loads of smaller interesting series, because nintendo does a wide variety of things. My roster, which is based solely on importance, adds more new series characters than characters from existing series. These characters include many popular choices like Ridley, K. Rool, Isaac, Toad, Impa, and Banjo. It also includes many very unique fighters like Andy, Excitebiker, Dark Matter, Daitoryo, and Inkling.