This. A lot of the early players of Splatoon will probably be Smash players. But I doubt that big names in the Wii U scene, like ZeRo or Dabuz, will ever fully switch over. They may buy the game, and perhaps play competitively a little, but they'll stick with smash. What I think we might see are some "small fries" from the Smash 4 community (specifically Smash 4, because they already own a Wii U) make the switch, especially if they have backgrounds in shooters.
This is assuming the game supports this kind of thing. I really hope it does... If not, I'm sure the community will find SOME way to compete. But it may not be optimal.
I REALLY hope players from other shooter games pick up this game. I don't expect them to do this on release, but if they see the progress the community makes and the potential the game has, they may give it a chance.