That’s the point - those mismatched packages often break the system. I had to do probably near a half dozen reinstalls after Ubuntu’s “clever” trick wrecked my system. I ran a Debian system from potato through to sarge updating each time with no trouble. My Ubuntu machine had problems virtually every upgrade (though most minor) and required more than a few full reinstalls.

The update won’t break the system if you follow the update instructions (remove packages from those repositories first). The Ubuntu way does break the system (see my other comment).

I don’t think so, because it shouldn’t be an automated process. Doing that blindly is a great way to have orphaned and incompatible package versions left on your machine.

I had absolutely no problems updating Debian to 13 from 11 to 12 to 13 one after the other. I also had no problems upgrading between Debian versions when I ran it as my main driver from the Potato release up until Ubuntu came out. Conversely, when I used Ubuntu from its original Warty release to around 2012 or so I had issues on literally every single version upgrade. Most relatively minor, but more than a couple requiring full reinstalls.

I would bet money that the vast majority of those having problems upgrading Debian are on “FrankenDebian” systems. Not all, but I am confident the majority are.