This is what DS shipments looked like by fiscal year, where 2008 means FY ending March 31, 2008.  In FY2008 they shipped 30.31m units and in FY2009 they shipped slightly more 31.19m.  That looks like what is happening last year (FY2021) and this year (FY2022).  This year is selling slightly higher than last year.  So lets continue this analogy.

The 3DS launched right at the very end of FY2011 (Feb/Mar 2011).  I put in the next fiscal year to show that DS sales were obviously dropping like a rock at that point.  This means that if Switch sales actually do fall off a cliff, then the earliest release for a Switch successor is Feb/Mar 2024.  A 2023 release date is unrealistic even in the most pessimistic scenario.  And if the Switch has gradual drops in sales, then it will release holiday 2024 or later (most likely later).

Nintendo will most likely bundle in games first before they do any price drops.  But they have a lot of time left to first do bundles and then do price drops. 

Usually, video game hardware does not generate much profit.  However, the Switch is different.  It was already sold at a slight profit at launch, and it's been on the market at full price for so long, that it has to be making them a lot of profit per sale.  They can actually do bundles in 2022-2023, and then start with price drops in 2023-2024 and still make a ton of profit.  I think it is unlikely for them to leave this much money on the table.  One big difference between Nintendo and Sony is that Nintendo has a lot of evergreens.  Every Switch sold is going to lead to quite a few evergreen sales.  Playstation games end up discounted at Gamestop (and similar stores) pretty quickly, so Sony doesn't necessarily see a cheaper piece of hardware turn into a lot of software revenue at the tail end of a system's life.  Nintendo evergreens keep selling though.  So, a $50-$100 price drop in 2024 is going to generate a lot more profits for the Switch both in hardware and in software.

Overall, I still can't say when the Switch successor will release.  I think it is too early to tell.  I know Switch is going to sell well in 2022, but what about 2023?  If it has a big drop in 2023, then I think Switch 2 will get a 2024 release.  On the other hand, I think it is more likely that Switch will still sell 20m+ in 2023 given how much room Nintendo has for bundles and price cuts.  If it does that, then I think Switch 2 will release in 2025 or later. 

Nintendo's business side is very conservative, and they are down to one main system now.  Given how much money the Switch is making them, I just can't see Nintendo being eager to kill off the goose that is laying the golden eggs.

2022

- Open World Pokemon

- New Zelda

- Splatoon 3

- Metroid Prime 4?

- Mario+Rabbids 2

- Some Major 3rd Party Game

- 2D Mario?

- New Switch Model

2022 I choose you! Since Switch (4k addition) plus all the 5A games that will cometh out :P

I don't think FY 22 being higher than 21 is a given at this point. That would be in amongst the most pesimistic scenario, but I see your logic

Are we talking purely hardware sales? Or also software?
Is it base units only? Or revenue?

Switch hardware units sold in the calendar year.

EDIT: edited the OP to include this clarification.

I'd say 2022 due to the software and potential upgraded Switch model incoming.

I had predicted 2021 last year! but who the hell knows at this point?
I betted on 2021.

if it's later than 2021 oh my godness.

It has to be 2021 right...right?