I don't see why it'd have to be integrated into apt-get itselfThat's exactly what I'm talking about. To give just one of many reasons: Because currently 'apt-get update' fails hard as soon as a PPA repo disappears. These corner cases must not exist. This is fundamental infrastructure, save the "easy workarounds" for higher levels.
Apt has been "almost there" for about 10 years now. But "almost" is not good enough - neither for desktop-users, nor for professional deployments with puppet/chef that have to fight the same and similar issues (e.g. idiotic license seed files, key conflicts, daemons auto-starting after installation, unreliable exit codes/error reporting, insufficient logging, insufficient hooks to override defaults).
We're in a state of inertia where everyone is so used to working around ever the same bugs that they're not even recognized as bugs anymore. I should absolutely not need a ~80 line puppet manifest or chef cookbook to make apt behave right. Desktop users should absolutely not have to find a bash-script (no matter how small) to be able to seamlessly install third party software.
If you want to have the ability to pull updates, you need some location to query for them, and that location is the repository. Even if it's just for a single package. If you want to install a single package without updates, there's deb files.
I'll quote pg on this: If you think something is supposed to hurt then you're probably doing it wrong.
Apt is a beautiful concept. Sadly the implementation stopped evolving shortly after becoming "good enough" rather than proceeding to "the best we can possibly make it".