If your logic was true, then they would have done the same with the last system that sold insane, the DS. It released November 2004 and sold over 150 million copies. It lasted 6 years and 3 months before being replaced by the 3ds. Why didn't Nintendo delay releasing the 3ds, if your logic holds up? Why not keep pumping out games for the DS in 2011 and 2012 and beyond, like you suggest the Switch should do with lasting until 2024 and beyond? The Switch won't pass the total sales of DS ever, let alone in the ridiculously quick time-span DS did. It's a fantastic selling system, one of the best ever, but it isn't some unprecedented performance that will change what Nintendo has been doing for decades.
DS was replaced in the third year after it's peak year. Which would have happened with Wii as well, but Nintendo pushed back WiiU a year, after it's lukewarm initial reception. PS4 was also replaced in the third year after the peak. The general logic isn't about the overall longevity of a system, but if it is already on a downward path. Switch so far could still peak in 2021. Or with the strong lineup even in 2022. But let's assume, it peaks this year, this would mean a 2024 release of the successor. I guess that will still happen. Nintendo will look if sales still rise or are decreasing and decide the release of the next system based on that.