I mean, the Switch wasn't made because of the people who bought a Wii U. It was for the people who didn't. (and it's quite successful at that from all reports)
An hypothetical 3D World port isn't delaying or "taking away" an equally hypothetical original Switch game. Infact, the profits made from such a guaranteed money maker could go toward financing more risky projects. When Zelda Wild Waker HD happened, it didn't delay Breath of the Wild or kill an original Zelda game, they simply outsourced it to a third-party specialized in doing ports. And personally speaking as someone who owns a Wii U but skipped 3D World, I'd gladly get a technicaly superior and truly portable version of the game.