There's the "Pro" argument, that we might get a half-gen refresh, but with the Switch's success so far, I doubt that will happen.
Streaming is at least another gen away (if not more). Ditto VR, if it catches on.
Soooo...really, it looks like their present niche is positioned to keep working for another few years. 2022-ish, they make a Switch 2.0 that basically allows for portable PS4/XBone performance, with enough modern GPU features to get a few (but not nearly all) of what will be "current gen" PS5-level games. Nintendo makes their usual exclusives. Third parties port older games.
The only question is if Nintendo is too restless with the idea of essentially running it back.