The "Switch sales will drop off now they've used most of their big IPs" theory has been going since late 2017, it was wrong then and it's wrong now.
It's also a mistake to assume Switch will necessarily follow the sales trends of past Nintendo systems has it is quite unlike any of its predecessors.
You can have that luxury because really if you're wrong it's not skin off your back, you can just say "oops" and move on with your day. if you're running Nintendo you don't have that luxury you have to take into account not everything is always smooth sailing and have a plan for that scenario.
And how is it a theory from 2017, Nintendo has several large selling IP like Pokemon and Animal Crossing and B-tier stuff like Fire Emblem that weren't available in 2017 or 2018. Even the GameCube didn't peak in sales in year 1, so that's not really a relevant point.
They need to start planning for Switch 2 really like now. Estimating a 3 year cycle for game development, you need to know what you're doing now.