Huh? This is exactly what the OP is saying haha. "probably" will reach 100m?? It will probably reach 100m NEXT YEAR! It should end next year right around 100m. And the Switch 2 will still be multiple years away at that point. 120m is like the guaranteed bottom at this point. Like, if Switch died HARD and very soon it would still sell 120 million at this point. Again, this is what the OP is talking about, why do people still say stuff like this?
Maybe it's the talk of the new Switch model and some people seem to get confused and for some reason think it's a Switch 2 and not just another Switch model that is coming out soon. Switch 2 is yeeeaaaarrs away, 3-4 years away at this point. And in just over one year from now Switch should hit 100m. That's a hell of a lot of time to sell above 100m before the Switch is even replaced.
If Nintendo replaces Switch a little on the early side, like say holiday 2023, Switch should be looking at over 130m lifetime sales. If they replace it at what I think would be the perfect time, Spring 2024, it'll probably end up around 140 million. If they keep putting out good first party games for it in 2024 and don't replace it until holiday 2024 and the price is all cheap by then, then it could hit 150 million (49+29+24+17+14+9+5+2+1 = 150).
Yours is just a theory of many. There's no evidence for anything you just said. At least I was basing my assumptions off history.
Wii had 6 years before WiiU came out. In case of Switch that would be March 2023. That's in 2 1/2 years. Certainly not within your 3-4 years estimation. On top of that I proved how badly the Wii sold in 2012, the year WiiU launched. Only 5 million sales. WiiU was announced 1 1/2 years before it came out. Using the same time spans, Switch 2 will be announced in Fall 2021. That means Switch would have only 12 months of strong sales left and then could drop off completely in 2022.
Btw. the successor to N64 was released after 5 years and the successor to Gamecube was released after 5 years. Finally, the successor to WiiU 4 1/2 years later. Yet you expect 6 1/2 years to 7 1/2 years for Switch.