This is very gross disinformation and/or incompetence (which explains why "others" won't listen).
The default Debian tools, apt and apt-get (and related tools, like gdebi/aptitude) won't put the system in an inconsistent state, unless the user forces them to do so. Dpkg will do, which I suspect is the tool you're using.
Even when one adds a repository with incompatible package versions (say, the system is Focal and the repository distribution is Mantic), the upgrade will stop before upgrading the packages.
What's happening is that you're forcing broken dependencies, likely to chase the "latest and greatest versions", then complaining that the package manager is broken.
Uh... Such is linux life, when you're hit with issues and complain, it's "This is very gross disinformation and/or incompetence". No wonder some people won't bother dealing with it.
Again, I truthfully experienced all of this and my day job is to handle things like this, so I personally know that it's neither of those things.
What's happening is a variety of issues (not one) and it can be anything from package maintainer not knowing whether a version is incompatible with their package, to dependency being noted wrong in package. For example, if I write package X and depend on numpy, but don't realize numpy 2.24 breaks it, Debian will happily accept the update breaking my package for users. It's the maintainer's fault but ultimately if the OS didn't upgrade dependencies for every program it would have worked. Which is why freezing dependencies is the way to go in terms of stability.
There are countlessly other scenarios, for example using a program compiled outside of Debian and dyn linking then Debian upgrading the dependency not knowing about the 3rd party program. It's all a trap for broken software.
The default Debian tools, apt and apt-get (and related tools, like gdebi/aptitude) won't put the system in an inconsistent state, unless the user forces them to do so. Dpkg will do, which I suspect is the tool you're using.
Even when one adds a repository with incompatible package versions (say, the system is Focal and the repository distribution is Mantic), the upgrade will stop before upgrading the packages.
What's happening is that you're forcing broken dependencies, likely to chase the "latest and greatest versions", then complaining that the package manager is broken.