Two ways to look at it. Of course, The Steam Deck is not a threat now.
But:
1: If these handhelds are becoming a way to play, Xbox Games, PlayStation Games an PC games on one device. One might just eat that loss and stay on a PC handheld and forget Nintendo.
2. If Nintendo really maintains first strong Party Support, there will always be a place for Nintendo Hardware.
It will be interesting to see how all this shakes out in the next decade.
Is Nintendo going to do a trifecta of sameness or will they have to find a new angle for the next console?