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).
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