PS1 and PS4 each did it once, the PS2 and the Wii each did it twice, but the DS shipped >20m units for 4 years, two of which were >30m. While the Switch never had a year that high, it will certainly ship more than 20m units next year and I do believe that it could still be shipping over 20m units for fiscal year April '23 - March '24.  I truly believe that it's possible at this rate, and here's why:

1. Switch is still on fire. 

I believe that 2022 is going to be massive for Switch.  In fact, if it weren't for the extenuating circumstances that boosted 2020 to peak sales, I believe that 2022 could have been Switch's peak year.  We have three 10m+ sellers with Pokemon Legends: Arceus at the beginning of the year, BOTW2 likely at the end of the year and Splatoon 3 somewhere in the middle.  Factor in other games like Bayonetta 3, new Rabbids, 3D Kirby, Project Triangle and so many unannounced surprises like possibly Pikmin 4 (and still hoping for the new DK) to go along with the hype of the OLED.  Normally, the only thing preventing Switch from 25m+ sales IMO would be saturation setting in, but since I firmly believe that 50m+ Switch units will still be sold in its life, saturation isn't really a massive factor yet.  However, there is one thing that will likely prevent Switch from selling its full potential in 2022...

2. The Chip Shortage

Frankly, if it wasn't for this, I think Nintendo could meet demand for 2022 and 2023 would fall below 20m units.  But with Nintendo unable to make enough Switch's to satiate demand, I believe that sales will level out, making spillover demand continue in 2023 for >20m units shipped.  Of course, 2023 would still have to be a great year for Switch.  There are price drops up Nintendo's sleeve, software we can hope for like Metroid Prime 4 and a new Pokemon sequel to Sword/Shield, and quite possibly one other thing that will give Switch its last hurrah...

3. New Nintendo Switch (4K)

Normally I wouldn't believe it but there are just too many rumors from reasonably credibly sources that suggest that a Switch with a new chip is coming.  Not a massive performance jump per se (probably very similar to NEW 3DS), but with Nintendo's partnership with NVidia it is very likely that the ability to do 4K when docked via DLSS is in the cards for a redesign.  If a New Nintendo Switch debuted in 2023 it would be enough to fuel another 20m+ year for the console in its then 7th year on the market.  Granted, it will probably be just barely above 20m, but I think that Switch will be the first game system to ship more than 20m units for five years. 

I want this to happen, it's not very likely though.

I think at this point, if a powerful Switch comes out like in the rumours, it will be a successor to the Switch and not a Switch "Pro" situation like you are suggesting. 2023 for a Pro revision will be crazy late as devs would have to support the base model.

By comparison, a PS4 Pro came out just 3 years after the base PS4 and One X came out 4 years after Xbox VCR. New 3ds came out 4 years after 3ds.

Are we talking Fiscal Years? In that case it is currently at 2 >20m, and will be 3>20m when this year is done. Next fiscal year can still hit 20m, but then that's it. so >20m for 4 years not 5.

It's not going to ship 20m+ from April 2023 to March 2024.

I don't even think next Fiscal Year is completely guaranteed, only needs to drop 17% YoY to miss that, and as it happens 17% is exactly how much shipments have been forecast to drop this year.

I can see it for 2022, because of the very strong software lineup and that the demand can't be matched this year.

We should see a price cut in 2023, but I don't think that this alone would help to save another 20m+ year. It would only be possible, if a (more powerful) Switch 4K would be released in 2023, but I don't think that this will happen at this point of Switch's life cycle. If rumors are true, the 4K dev kits are not able to play Switch games on-the-fly, so it's more likely that the new Nintendo hardware is rather a successor than another Switch model. I expect a new Nintendo console in March 2024. I don't think that they cut off the sales of Switch too early with a successor, because of the experience with DS/3DS.

24M is the FY prediction by Nintendo. I expect next year to be

I personally am one who thinks Nintendo wont quite ship 20M in 2022. I think it will be close (18M-19M), but I don't think they will quite reach it. I accidentally clicked on the wrong option though lol. However, I do think it's a possibility. What I don't think will happen is 20M in 2023. I think that year we are going to get a decline to around 11M. So I'm thinking it looks like this:

end of 2021: 102-103M
end of 2022: 120-122M
end of 2023: 131-133M
end of 2024: 137-139M
end of life: 141-144M

To be what?

And still over 140mil. How are there so many people who think it'll end up below 130mil.

It's that cliff man. It's going to happen ANY day now.