Instead, currently, all packages become lint-unclean, all maintainers need to read up on what the new thing is, how it might break, whether/how it affects them, manually run some tests, and finally decide to opt in. This causes a lot of overhead and manually executed mechanical changes across packages.

I always wondered if Debian/Ubuntu could benefit from a "monorepo". It seems to work for other distributions, e.g. Alpine Linux and Homebrew.

https://github.com/alpinelinux/aports/tree/master/main

Right now every Debian package lives in a separate repo, or it doesn't even have to live in a repo at all AFAIK.

I think Debian has the most packages because their process is very loose and decoupled (as well as it being one of the oldest distros). But having tighter integration does help move things forward faster.