But what's the problem with sources.list, as long as the user doesn't have to manage or even know that it exists?

Provide a .deb from your website, use the install script to add your repository to the sources.list. There, the user doesn't have to know or care about that file. And this is all possible - no, easy - to do right now.

This is how it must work:

I disagree, having to run apt-get is too cumbersome to a regular user. But a better way already exists in the form of apt-url. Click a link, have the package downloaded and installed automatically.

This can (will and does) co-exist happily alongside the centralized repositories. Someone just needs to implement it and push it through the glacial Debian processes.

But my point is that most of the infrastructure (support for multiple repositories, one-click installation of third-party packages) already exists. That's why I don't agree that this is the problem with Linux.

And while we're at it, there's no reason 'apt-get install github://foobar' can't be made work.

I don't see how - there's no standard on GH projects for installation; some projects are installed by simple make/make install, others with easy_install/gems/npm, etc.