That only works on the 2 or 3 buttons that are hard-coded to support it and doesn't work in the general case (it can't work in the general case because it's literally just a long press, which already does other things).

Personally, pushing on the flashlight or camera buttons with 3D touch actually feels really satisfying, because it really does feel like I'm clicking a button even though I'm just pushing on my screen.

I suspect Apple may go that way, but, for example, long press and 3D Touch on a folder in springboard do different things, both useful.

That’s long press and even longer press on my iPad.