Yeah, the answer is in that thread, I think @pradyunsg has been champing at the bit to actually write a PEP for this (I was hoping for a proof of concept first, but if we’re at the stage where we need someone to implement it, maybe we should just do a draft PEP waiting on implementation). A simple summary is in this post.
A lot of the devil is in the details, but I think one of the key issues that makes this “dumb” hook unacceptable to me is that it blurs the line between frontend and backend in a way that we’ve been explicitly trying to move away from. This requires backends to know how to install the things they build. The proposed standard in the other thread was pretty simple: the build back-end prepares a “virtual wheel” that maps location-in-wheel to location-on-disk, plus the wheel metadata, and hands that all over to the front-end for installation. This is a clean separation of concerns between front-end and back-end, because the front-end doesn’t need to know anything about how to build the package and the back-end doesn’t need to know anything about how to expose packages to the system.