You're right that they do have a history of cutting off their portable systems very early (GBA, DS), but not their home systems. Seeing as how they only have one system now I don't see them cutting off a system that will deliver huge software sales for years to come.
We also have Nintendo having repeatedly stated that they are planning for a longer lifespan for the Switch, which means no handheld-style early cutoff is happening.
We're four years in, sales will probably be about 30 million this year, we've only had one model variation and waiting for the second one, and zero price cuts. The Switch train ain't being replaced any time soon.
I suppose if they did decide to cut it off early, so as to not lose any hardware momentum, they could do a holiday 2024 or Spring 2025 successor. Switch sales should still be very robust in 2024 and then they'd go straight into having a new system on the market. The people who are talking about a 2022/2023 successor are nuts though.
Depends on their strategy. If they want no loss in momentum but willing to sacrifice many tens of millions of software sales on Switch and maybe 20 million hardware sales of Switch they could replace it around holiday '24 / spring '25. Or if they want to milk its popularity and user buying enthusiasm and let the upgraded 4k model replace the original model and bring out a new Mario Kart on Switch and do another 3D Mario and so on and fill out all the missing IP that Nintendo has then 2026/2027 would work (those sorts of games coming out in 2023/24 and then ready to hit Switch 2 with the next iterations in 2026/27/28. That way they could maybe have a powerful enough Switch 2 that can get multiplat third party games with the 2020 home consoles.
The fact that the rumored new model is 4k capable almost gives credence to the idea that a new system isn't coming any time in the next few years. Switch 4k model bridges the feature gap between Switch and the home consoles/Switch 2. So it makes a Switch 2 anytime in the next 3 years seem far fetched. They can sell the 4k Switch model at $400 when it launches in '21/'22, and for people who think the price is outrageous that's fine, the OG and Lite models are available. In '23/'24 they can drop it to $300 and still have a full priced system selling and get rid of the OG model. So that means going into 2024 there is absolutely no need for a Switch 2 at a similar price point to the Switch.
They did cut off the Wii kinda early but their intent to have a longer lifespan for the Switch is good evidence for the chance of that happening. I just find it a bit bothersome when people say the high sales means a really long lifespan when that hasn't been the case multiple times before. I wouldn't say holiday 2024 is early since that'll mean the gap between the Switch and its successor is a bit over 7.5 years which would be longer than every other Nintendo console gap other than the GB and GBA one but I do agree thinking 2022 is possible is really silly since if that was happening there would already be rumours and whatnot going around about it.