Overall, I think there are many technical, cost, and project management arguments against 3D Touch. On the other hand, the arguments for 3D Touch are largely marketing and UI based. Apple hasn't promoted 3D Touch in a while and iPad Pro proves 3D Touch is not absolutely necessary.
Personally, I won't miss 3D Touch too much. It is useful, but long-press can be substituted in many cases.
I believe this could be a factor as well. The 6.1" iPhone display area is roughly 20% larger than iPhone 8 Plus. Yet the battery capacity is less than 10% larger. It also needs to power the TrueDepth camera. Unlike iPhone X, Apple is probably unwilling to use expensive production techniques like a stacked logic board and L-shaped battery to gain capacity.
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We know 3D Touch adds thickness to the device. This is measured to be about 0.5mm when using a caliper to measure the thickness of iPhone 6 vs 6s displays.
0.5mm may not seem much, but the iPhone 6 Plus battery is only 3mm thick with 11 anode/cathode layer pairs. Below is an actual photo for the 6 Plus battery. This thickness translates into a loss of volume for the battery.