I actually was one of the first people who noticed these issues, and I had a similar sentiment that we should fix all of them semi-automatically before we released. However, the qt wrappers were not a breaking change in the first place. If you read the threads it is because a huge majority of packages were written incorrectly, often not using the qt specific deriver and callPackage function. Because of this it would be entirely unreasonable to ask one single person to fix every single expression for the maintainer who didn’t package it correctly in the first place.
I talked with numerous people on IRC in #nixos-dev and Thomas Tuegel as well, and we got a change that would abort builds that don’t use the supported deriver https://github.com/NixOS/nixpkgs/pull/70691, making all applications that fail at runtime to fail at build time. Then you could easily run a jobset and do a sed operation on all the newly broken packages. The issue for me is I didn’t really have the time to really support this, so I’ve done my very best to leave a very good trail so people can help. In the meantime, on a case by case basis I’ve reviewed probably 100+ submissions to fix expressions and made sure they were backported to stable releases always.
We are going to actually mark every package that doesn’t build as broken at the time of 20.03 release. But things not working at runtime is something we don’t have the infrastructure to determine automatically.