It's not completely impossible - right now it is selling on pace with the Wii, and I think there's enough reason to believe Nintendo will support the system for longer instead of dropping it out of nowhere like they did with the Wii (because Nintendo always supports handhelds considerably longer than home consoles, and unlike the Wii, the Switch can play any sort of regular, traditional game without additional accessories, which means they don't need to restrict the kinds of games they want to make, which I think was ultimately the reason why they dropped the Wii so fast). With those things in mind, it's not out of the question that the Switch could sell up to 120m or even more... I just don't see it happening, though.
Plus, the Switch is going to be the one system they're developing for going forward, as opposed to dividing their resources between two. Sometimes between three to four different systems when they were preparing to launch a new handheld and console. (N64 and GBC -> GCN and GBA; GCN and GBA -> Wii and DS; Wii and DS -> Wii U and 3DS)
Now, two systems will be the maximum that they develop for (three if you want to count mobile.) This means they will have more resources at their disposal to help ensure a strong launch for their next system (hopefully, NS2) while quietly closing the door on their old system.
We're seeing that even now with the 3DS and Switch.
Even though the Switch had a phenomenal 1st year, the 3DS still had a pretty decent 6th year between Shadows of Valentia, Samus Returns, Monster Hunter, DQXI, and Pokemon. (I know two of those games are 3rd party, but still.)
Pancho A. Ovies
PlayStation 4/Xbox One/Nintendo Switch: 2019 vs. 2020