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.
Well this year and last year are just going to be weird to look back historically because of the pandemic. No time in the history of video gaming has there been such a prolonged period of either forcing or implying that people should leave their homes as little as possible. We are what, a year and a half since this all began, and still there is a large amount of the population that thinks people going to parks and restaraunts as the source of all evil (at least in the USA). That leads to a lot of downtime at home for those fearful to leave and honestly has, in my opinion, built a bubble up within the gaming industry. I think in 2022 and especially 2023 we are going to see a major gaming crash as people focus more on doing stuff outside in a massive pendalum swing. I think there will be considerably less people turning on their systems everyday compared to 2019, let alone the years of the pandemic.