Are you kidding me? Nobody in their right mind thought, in 2019, that the Switch was going to be just a fad. It was already selling a lot, software was doing amazingly well, including third parties, and unlike the Wii, the switch audience was much more versed in gaming. Nintendo was actually so confident in their machine that they often repeated that this generation would last longer than usual.
Releasing a successor after 7 years and supporting the current model for 2 years after that, giving the og switch a lifetime of 9-ish years, fits exactly in that narrative.