It's true that a lot of 3D Touch features can be implemented using long press, but there are advantages to 3D Touch. Firstly, for all applications the result is more immediate - there is no need to wait. And secondly you can vary the input after the initial gesture, which is useful in some situations.
For example, the 3D Touch gesture in Poison Maps (full disclosure - it's my app) was previously written using long touch. You would press and hold on the map and it would zoom out. It would keep zooming out until you move your finger to pan across to the required area. Let go and it zooms back in. Unfortunately you can't vary the zoom factor whilst panning because the initial long press gesture is over.
With 3D Touch the zoom factor can be varied at all times by pressing harder or softer. This allows you to zoom out a fair way, pan across to where you want to go, and then zoom back in a bit to get exactly the area you want before letting go. The zoom factor is controllable during the whole gesture, rather than just at the beginning, as it is with a long touch.
I think that shortcuts and peek/pop are just the start of the features that can be implemented using 3D Touch.