One thing I haven't seen mentioned is how well Nintendo's other handhelds have performed. Here is a brief rundown of each of Nintendo's handhelds by global sales
Keep in mind too that the GBA was cut off early so it was only on the market for about 4-5 years. But as you can see, the average for Nintendo's handhelds is 107 million. This is already about where the PS4 is at. Switch only needs to sell a bit more above the average which is very doable. Nintendo's Q3 results aren't out yet but so far Switch sales are up about 30 percent through the first 6 months.
I think this assumption that it can't happen is really driven by people looking at the console side and not the handheld side. There is a reason Nintendo made a hybrid system.
GBA was cut short but on the other hand Gameboy had an insane sales curve thanks to Pokemon & GBC causing it to peak in its 10th year and went 12 years before getting a successor which is unheard of so I think they cancel each other out.
When the herd loses its way, the shepard must kill the bull that leads them astray.