That's a good point, though a comparation with Wii/GBA which did >7m in the 4th year would be hard to reach, but surely would make more sense than 3DS which did 2.57 million.
I don't think that would be hard to reach, actually. So far in 2019 Switch is going at a pace where it has a realistic shot to sell 7m for the year and having another year like that isn't too farfetched when the timing of price cuts and revisions is taken into account.
2020 is also the year where the development teams of Mario Kart 8 Deluxe/ARMS, Splatoon 2 and Super Mario Odyssey can have their next games ready, so there's no good reason to expect a shortage of system sellers. The developers who finished 3DS titles in 2017 and 2018 are also due for new releases and while not all of them will make it by the end of 2020, there will be more robust first party support for Switch in its fourth year than for other Nintendo consoles in the past, simply because Nintendo has only one console to support this time around.