Motion-heavy games have a place on the Switch, but certainly not in the driver seat (as with Wii) or even in the front passenger side (as with Wii U), but firmly in the backseat once in a while.
You're probably right that they exist as tests for whether a motion control audience still exists, and how large it is.
I'd imagine "optional motion controls" like Zelda, Splatoon, and apparently Skyrim is going to be the standard on Switch. It is with ARMS as well, but when played with a regular controller, it's not that riveting of an experience.
Ultimately I think the whole idea of motion controls is moving in the right way. Motion controls are now available for the cases in which they're actually preferable now, like aming a reticule, or rotating an object you're holding. There was a five-ten year period with PC as well where all games had to support both keyboard-only and keyboard+mouse options. And during the PS1 days analog stick control was mostly optional. Maybe one day will will all agree on what motion controls to keep and which will bow to history.
On the case of ARMS though, I think it will keep selling reasonably, and that a sequel of some form won't be out of the question.