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.
Switch is getting an upgraded version for next year, I don't see Nintendo teasing Switch's follow up before mid-2022