Amen!
3D Touch has been an obscure power user feature for hard to find features and a gimmick for easy to find features.
And it’s resulted in phones with smaller batteries, that are heavier and as you said, thicker because of this (compare the 7 and 6s with the 6).
It’s hard to imagine that this was the major feature of the iPhone 6s series and that tens of minutes were spent in demoing this onstage!
Importantly, like many others on thread have said, I think that Apple wants to make the iOS experience on the iPad and iPhone, consistent and that can’t be done if one device has 3D Touch and then other doesn’t but had stylus support.
From a supply chain POV, I’d assume that Apple want to get into the place where by 2019 all of their iOS devices are using the same LED and OLED screen parts.