I went 2020 last time. Could still turn out to be correct, but I’m leaning towards 2021 barely topping it.

It's got to be 2021.  This year is already tracking ahead of 2020, which means it is doing better than the Animal Crossing + beginning of COVID lockdown months.  With a new hardware model + Pokemon, Switch is definitely going to be higher this year.  At the same time, the momentum this year is only slightly higher.  Switch's sales growth was really rocketing up in 2019 and especially 2020.  It's not doing that this year, so I think this is going to be the peak year. 

Next year will be down slightly, but it will still be high (probably 25m+) due to a bunch of solid software releases.  That is why anyone saying Switch 2 is coming in 2023 just has no clue.  Imagine Nintendo saying this in 2022.  "Hey we are having one of the best sales years that any system has ever had.  Why don't we kill off this goose that is laying all of these golden eggs...."  Nope.  That ain't happening.  So while I think 2021 will be Switch's peak year, I think every year 2022 and on will only have mild drops in sales giving the Switch a very long life.

I think it'll be 2021. This year I think will sell a little bit more than last year, maybe 29 million.

Next year, with Pokemon Legends, BotW2, Splatoon 3, and who knows what else, I don't think Switch will lose much steam, it very likely could do another 25+ million year next year, but I suspect it won't quite be able to match 2020 and 2021 numbers. Now if they throw out a price cut and like a new 2D Mario game to really pack the year with mega sellers then all bets are off. So I won't put it totally out of the question that 2022 could be the peak, but I think it'd only happen if Nintendo got really aggressive with trying to push sales by actually doing things, which isn't really their style.

Mostly agreed. Nintendo could keep Switch going to like 2026 or so if they wanted to. But I suspect they will sort of cut it off, not in 2023 obviously, but perhaps launch Switch 2 in holiday 2024. I'm thinking Nintendo might not even bother ever discounted the Switch and Switch Lite. Didn't they not really ever even drop the Wii U's price despite the terrible sales, or maybe dropped it $50? Nintendo doesn't seem to like to make business moves to sell more units, they just like to keep prices high and let everyone buy at those prices then move on to the next system.

Switch OLED will allow them to discontinue the original model and drop OLED to $300 eventually and keep that price for a good long while. They COULD drop the prices of the systems in 2024 and 2025 and 2026 and keep the thing selling and maybe even launch that rumored but false 4k upgrade model to also keep a higher price point in there and keep excitement for the Switch going well in to the middle of the decade. But I see them not bothering much in the way of price cuts on anything and just allowing the market to saturate at the initial launch prices and readying Switch 2 by end of 2024. That way they have a new system on the market just after Switch sales start dropping below 20mil a year (I figure Switch will drop under 20mil in 2024, with or without a successor launch. Could happen in 2023 but Switch might be just over 20mil in '23) so that they don't have any low hardware years since they don't have a second system to back things up.

I voted next year but I'm still of the mindset that Nintendo releases a Switch Pro as a possibility. They've stated multiple times that this is the middle-ish of the Switch lifecycle so I assume there will be another revision and the only logical thing I think they can do now is make it more powerful.

Isn't this exactly what happened in 2010? 
FY ending 2010 DS sold 27m
FY ending 2011 the 3DS was out in Japan (shortly to follow elsewhere)

I voted later than 2022. Gut feeling. We already know that 2022 will have quite some big games so I expect the momentum to keep on going strong. Still no clue about 2023 but I can't see them just dropping the ball for no reason. I guess that 2023 will have some more big games as well, because why shouldn't it. In the meantime the competition probably will have established a solid library and so I think that is when the Switch sales will start to slowly descend.

Call me crazy, but I think the Switch Pro is still coming, just next year, not this one. The OLED model is a half step and will replace the base model, while the actual Switch Pro launches in 2022. Basically a 3DS XL model vs a New 3DS model. Nintendo has shown that they are more than happy to release multiple revision within a single generation. Between that 'Pro' revision and the heavy hitting games that year (plus the chip shortage being hopefully resolved) we should hit 30 million.
Now, this does hinge on a couple of things: first that the chip shortage eventually catches up to Nintendo this year and prevents them from tracking too far ahead of 2020. Secondly, that the new dock contains the technology to allow 4K upscaling that the Switch Pro is rumored to get. The old dock, if I understand it correctly, doesn't have the ability to handle 4K bandwidth, even only upscaling rather than native 4K. Data miners found references to upscaling in Nintendo firmware which makes me think Nintendo is setting the stage for stronger revision.
But I will confess, this could be more hopeful conjecture than well-founded argument since I definitely want a Switch Pro.

I ignorantly picked last year without knowing what’s up in 2021 so far :). I personally think the games this year are dismal, but the clearing-up of covid-19 is not having the dampening effect I thought so far.

edit: a good reminder that the gaming business is momentum-based

I voted 2020 because I think late summer and early fall will be tough for Switch this year to top the performance it had last year. Though I bet for 2020, I realized something: we're talking about calendar year and not fiscal year and it's a HUGE difference. Last year Jan and Feb were slightly down vs 2019 but those months are in the final quarter of FY, while for CY they are the first one, so it should be an advantage for 2021 Switch. I think in the end it will be slightly down YoY (like from 28m to 26.5m/27m), and I think Switch OLED won't change that much, probably 2022 will benefit more than the last months of this year. We'll see.