Mainly because Nintendo consoles seem to hit a wall hard ll of a sudden. This is partially Nintendo's fault as they abandon the console and don't continue to make games and focus on the next.
Wii at one stage felt like it was tracking to hit 140 million, then suddenly sales died.
Yep, in the past, Nintendo needed to refocus it's Software efforts towards its next system and had to do this while balancing Handheld and Consoles software lines & hardware launches. Switch has resolved this issue until a successor is released since it has combined Nintendo's software output.
With the Switch, past trends are really out of the window. The Switch will not perform like prior Nintendo systems in its price bracket, so any illusions about falling off a cliff or slowing down are a bit of wishful thinking from people. Next year will likely see hardware sales that rival this year's, and the Switch is yet to have an official price cut, Switch Lite being a somewhat of a price/feature cut(similar but more successful than the 2DS).
Some people underestimate it because they are only interested in Nintendo failing.