From what I've seen, if you own either console you should own this game... so I'm not sure that 10 million is out of the question. Zelda has never been a launch title before, and I really think it will have about the highest attach rates with the Switch that we've seen from Zelda given the lack of alternatives.
Otherwise, people with Wii U's have been waiting ages for this one. 10 million seems plausible over the coming couple of years, especially as past Zelda's were often hurt by coming out later in their console's lifespans.