I wouldn't say "the cliff". It's just perfectly normal drop. Past year 5, each year the YoY sales (on average) drop by 20-ish%. The biggest shot in the dark is 2022 perdiction. If that turns out to be true, then 140M lifetime is pretty much given.
Sure it's not going to drop as quickly as Wii, but more like the DS ... from 23M to 14M, also in year 7. To avoid it, Switch would have to have Gameboy-like sales curve and support.
A 20% drop YoY is relatively normal, but why are you suggesting a 40% drop after that? I mean, to contextualize something like the DS, it's YoY drop was incredibly drastic due to the announcement and subsequent launch of the 3DS. Even when it came to the Wii the first drop was 20% YoY, the second was something like 27% YoY, the third was 35% YoY (year Wii U was announced), and the follow year was over 50% drop (year Wii U came out).
Just to put it into perspective, the 3DS was announced 5.5 years after the launch of the DS and launched 6.5 years after the launch of the DS. So the Switch 2 would need to have an announcement this year and a launch next year for it to follow a similar pattern, contextually speaking.