I think 3D Touch is for advance users. I am surprised apple didn't market it that way. Listen to my theory first. If you are a normal iPhone user you just use your iPhone like before, nothing changes. But if you are a advance user who is willing to put in more effort to learn you could do things much quicker.
Open the camera app and click on the front facing camera icon to switch the camera. Then take a selfie. This is how most users use it.
Or 3D touch the camera app to get a menu to go to selfie camera directly.
This is a clever way to add more complexity to iPhone, but still keeping it simple for regular users.