Being able to always rely on the package manager alone, in other words on the built-in repos alone, has never been achievable on a stable system. You have to throw stability out of the window to allow for that to happen. There are huge downsides to that, especially for new users who have no clue how to isolate and work around defects. That’s why sideloading content via third party repos or individual debs has always been a part of the reality of Debian-based OSes. As a result, most open source communities and proprietary vendors provide one or the other.