I really don't think Nintendo would do that. For one, they've already promised simultaneous release for both versions. Would be a bit of a jerk move to backtrack on that. And two, even if they did backtrack on simultaneous release, why would Nintendo release a new title on older hardware first, then on their newest offering second (and months later, no less)?
Heck, just go back to Twilight Princess: first on Wii to help drive sales on the new hardware, then on Gamecube for those who didn't plan on getting on a Wii anytime soon (or just couldn't find one). If simultaneous release isn't a thing, then we're definitely getting the Switch version first, then the Wii U version a few weeks later for any Wii U owners who are taking a "wait-and-see" approach to Nintendo's new console.