I dunno about "failed miserably". It may be an expense thing that they're trying to keep costs down on the Xr. Both of the new Xs iphones still have 3D touch. I'd probably rewrite this title as "A UX teardown of Apple's 3D touch and why developers have not embraced it." I use it quite a bit, but maybe I'm an outlier.
I think a huge reason it failed is because other than people who read tech blogs or people who are extremely interested in phones, there was simply no way of knowing it existed.
Force touch is sooooo good for precisely moving the cursor when typing...
I dunno about that. Especially with artist tools like procreate, and adobe products, it's essential for brush/stylus work. Anecdotal opinion of course, but I use it almost every day in music creation with instrument apps that take advantage of it. (Anything moog & seaboard)
I think having many similar options available to users (long press, right click, force touch) is the problem, and they can still make the force touch work if they want to.
Force touch could be the mobile version of right click on desktop. They just need to be ok other platforms using this (Android) and disable right click on desktop, replace with force touch which will grow the usage on mobile too (adaptation). ☺️