Why make a Switch 2 in 2 years when they can do it in 5 or so years when the same tech out in 2 will be much cheaper giving them more control on pricing, right now a traditional type of handling makes more sense. A NSW2 in 5 years that hovers around PS4 level of performance on the go should be afforadble and still a large jump than releasing upgraded model every 2 years.
PS4 tier mobile chips will be available by next year actually. The tech is moving very fast. Nvidia has already finished the Parker, which would be the successor to the Switch's Tegra X1 ... they are now well into development of Tegra Xavier, which will debut in 2019 or so. The Xavier chip will give you PS4/XB1 tier graphics.
The model ranges may be different Switch 1/2/3 may not be it, you might have Switch Pro, Switch Alpha, whatever.