Zelda never achieved 10M sales, even on the 100M Wii userbase. I don't think it will achieve that on the Switch either. The attach rate will be big, no doubt about that, and it will sell really well, but I don't think 10M people are interested enough in it to buy it.
User base isnt everything. It is an equation to it but look it pokemon gen 4/5 vs gen 6/7 for example. (DS vs 3DS)