This might be a bold statement, but I can honestly see the Switch getting at least close to DS levels (like 120-140 million), or at least follow a similar trajectory to that machine. The Switch is a monster in Japan much like the DS was there, and again, its handheld functionality and multiple models mean there's added value for many in addition to having the potential for many households or even many people to purchase multiple consoles. Nintendo and 2nd parties just has to keep the games coming, and really the console already has an insanely big and diverse lineup for only being on the market for 2.7 years.
I really don't see the next gen of Sony and MS consoles effecting what Nintendo's doing too much, again, aside from less multiplat games from 3rd parties, but those haven't been a huge factor for Switch in the first place.
"We hold these truths to be self-evident - all men and women created by the, go-you know.. you know the thing!" - Joe Biden