It will be interesting to see how it plays out this year. I don't know if 2021 will beat 2020 or not, but I do think it will be close like +/- 1m. 2020 obviously benefitted from Animal Crossing and COVID. I think 2021 will have a lot more games overall including some really great ones. There is no chance of 2021 having anything that sells like Animal Crossing, but having several strong titles may have an even greater effect than Animal Crossing alone. Also 2021 will very likely have stronger holiday titles.
IMO, the biggest hurdle 2021 will have is the age/saturation point of the console. Once a system gets past the halfway point in total sales, then sales simply start to slow down. It doesn't even matter if the year has a bunch of great releases if most of the market already owns a system. So the big factor is really how many potential people in Japan still want a Switch. If that number is at least another 17m, then I think 2021 will sell more.
As I said though, I don't know the answer, but I do think the two years will have fairly close sales.