Well hopefully Nintendo will do regular price drops for their console, keep pumping out games at a good rate and building support. The 3DS suffered because a notable number of big releases stopped coming after a little whileAll good.
Switch shouldn't peak for a couple more years barring a 3DS-like sales collapse. I expect the PS4 to start declining next year since it'll be in its 5th year on the market. And the successor will probably out 2019-2020.
Hmm...maybe but I personally don't think we'll see any significant decline till the PS5 comes out. Until then, it should keep selling well