Switch is not the 3DS ... DS/3DS are declining and Vita died outright because they were too close to mobile gaming.
They're low end devices.
Switch is a portable console, that makes a difference, it's a higher end product. In 3 years IMO Nintendo will have a Switch that can run PS4/XB1 graphics/ports join their Switch family (Tegra Xavier), and they will not have the same decline in years 4/5/6 if they are smart.