Switch 2021 sales are hard to predict, and will depend on a lot of things.
First, i believe is almost guaranteed a pricedrop in 2020, the later in the year happen, the smallest will be the YOY drop in 2021. If Nintendo makes a pricedrop around July/August for example that's a lot of months to sell, while if they do a pricedrop in November, the momentum for Switch in the first half of 2021 is going to be very strong.
second, new models, there are many rumors about a Switch pro in 2020... i really don't know what to think about this, i honestly can see both 2020 and 2021 very likely for that new model. If Switch Pro is 2021, that's also gonna be a good thing for the next year.
And of course the line up. Is Breath of The wild 2 gonna be 2020 or 2021? Sinnoh remake 2020 or 2021? Metroid Prime 4 gonna be 2021 or ever later than that? What kind of Mario are we gonna see those years? We already know Animal Crossing will be at the start of 2020, and that's gonna be big expecially in Japan. But we'll see what kind of line up those 2 years are gonna have.
Lastly, pls stop making comparation with other Nintendo consoles. Having Wii and 3DS dropping badly after some years, most Nintendo consoles peaking in the 2nd year ecc seriusly mean a shit. It's been 3 years since Switch is in the market, and at this point it has to be obvius the trend is different.
Switch trend is looking closer to a Playstation/XBOX console to be honest, i really think comparations with those consoles is better. Surely it won't have legs as the PS2 or the PS1, or even the 360. But i think a similar trend to PS4 is realistic to expect.