On the one hand, you could argue that it's "doable" if you look at it as combining home and handheld sales they typically get together. If you look at Wii U as a badly marketed, badly supported aberration, then their typical console sales are something like 20-30 million, while their handhelds are usually quite a bit more, in the 60-100 million range.
BUT, you also have to take into account that the massive sales of Game Boy, were largely driven by two games: Tetris, and much later, Pokemon. And that the massive sales of DS/Lite/DSi, were largely driven by the so-called "Blue Ocean" era, wherein they had a lot of games like Nintendogs and Brain Training, etc., that appealed to non-traditional gamers. Same can be said for Wii, with Wii Sports and Wii Fit, etc. That Wii/DS era was incredibly good to the games industry in general, as Xbox 360 and PS3 eventually went on to sell quite a bit each, also (though 360 was admittedly buffered by the RROD phenomenon).
But the point is, that era is over. So looking at it another way, while you can certainly imagine that IF Nintendo actually displays signs of having learned their lesson from their BAD mis-steps in marketing and support for Wii U, and they actually get (and KEEP) third party publishers on board, then yes, Switch could be, potentially at least, a big success. HOWEVER, that's a lot of things going right and falling their way, to make that happen. And even if all of it DOES go THAT right, it's still a highly contentious idea, that Switch could sell over 100 million units, lifetime.
