I think 2021 will be packed with great games and the Switch will sell more in 2021 than in 2020.

Basically you are predicting that it will have a very rapid decline after this year, more rapid than the Wii.  I can understand this position.  I don't agree with it, but I understand it.  I am predicting a more gradual decline after this year.

Going by fiscal years, Nintendo shipped 21.03m for the FY ending March 2020 for a total of 55.78m.  Bloomberg reported Nintendo plans to ship 30m this FY.  I'll say it will be only 29.22m so that the total will be a nice round 85m for FY 2021.  Next year (FY 2022) I think they will ship more than 2020, so around 24m, which brings the total to 109m.  After that, with gradual annual drops, it isn't difficult to get to 160m-170m.  Also consider there is no way they are going to release a new system in 2023 if they ship 20m+ in 2022, Nintendo has tons of room for price cuts and hardware revisions still, and third party support keeps getting better each year with games like Monster Hunter Rise releasing next year.  It is quite likely that the YoY drops will be gradual.

The way I see it, the three main things that could pull them up short are replacing the Switch too soon, letting software support sag, and not being liberal enough with price drops.

While I don't personally expect them to reach 150 million, it's certainly not impossible, they'd just have to play their cards basically perfectly from here on out.

Yeah basically. They have to do things they aren't known for. If Nintendo is pumping out great software onto the Switch into 2024 before being replaced, has enticing price drops for hardware and software, and continues to support it in some way after the successor comes out, 150+ million is a possibility.

But my guess is Switch will end up at more like 135-140 million because first party software will be starting to run dry in 2023 as they are gearing up for successor launch in probably early 2024 and they'll switch over all development to the new system, and likely we won't see permanent price drops on older first party games until 2023 and we might only see one set of hardware price drops before like late 2023.

It's a question of how long they want to fully support the Switch and how they weigh profit per unit hardware/software versus attracting more people to buy with less profit per unit.

They should sell 149,999,999. Then they'll only have to sell one more!! Easy!!

I didn't make a prediction, just noting that 120-135 million (from Wman) isn't an unreasonable prediction, especially given Nintendo's past longevity.

The Switch itself being a hybrid handheld/home console it might perform more like the DS did, which declined faster than the Wii did, but that was after an almost 3 year long peak sales period.
At this point we just don't know... by April-May next year we will have some idea... if it's still selling 300k+ per week then it will be on track for a repeat stellar year lke this one, then 160 million is easy and 170 million well within sight.

^ that...

Expand the market with Erotic games;)

at this point I would say more re-releases, and when WiiU games run out, there are tons of Wii games waiting for HD versions, new features and more accurate motion controls

a serious joke, seeing how they sell re-releases with a few tweaks, they don't need much more than that

Yeah pretty much, evergreens will prevent a "cliff" scenario, but eventually they will have to shift their development teams over to Switch's successor, likely resulting in a dry last year or two before replacement.