Debian based distros should have a ".debs" file, and a program similar to gdebi, but for sources.list; for example.
Joe wants the latest firefox, he goes to firefox.com, downloads the ".debs", runs it. It asks for his password, and adds firefox.list to his /etc/apt/sources.list.d/ and also installs all (or maybe the suggested) the packages from said repository. Firefox.list contains firefox`s officially supported debian repository, and gives Joe firefox updates for the life of the repository.
Essentially, every application could supply their own repository; freeing Ubuntu from having to maintain these packages, and freeing software vendors from rolling their own update managers -- all at the same time as providing users with the latest software of their choice.