Nope. Nintendo will always be pulling out new gimmicks. This holiday season they’re doing Zelda controllers as the next gimmick.

I think with the Switch they've just finally figured out their ideal gaming format. Nintendo always crushed handheld, now they have a handheld system, that's also a console. Wii motion controls resurrected Nintendo's console division with motion controls, but was sort of a fad because only a few types of games are much better with motion controls, so they still have the motion controls in for those few types of games, but have standard button format for the vast majority of games which makes it easier on developers, while having the joycon controller setup that can switch between handheld and console play.

So there's no real need to continue to experiment with the controllers except add a little new thing here or there (Switch's IR sensor, S2's mouse mode). After many generations of experimenting they've come to the optimal controller design with the Switch systems. I doubt we'll see a major new experiment in the foreseeable future, just small side features.

So it's not about plateauing, it's that the experiments have succeeded in getting Nintendo to their ideal controller format.