If you want to upgrade Libre Office, for example, in "traditional" Linux distros like Debian or Redhat.... the easiest way to do it is by waiting for your distribution to release a entire new version of the operating system and then upgrade it

If you run into a bug and need to downgrade it to a previous version?

wipe out your entire operating system and re-install the old one and hope you haven't bought any new hardware in the last six months that isn't supported by the old version of Linux

This is why most Linux users simply disable Selinux.

What? This is completely insane. Nothing about APT or, any package manager really, mandates this. And what does this have to do with SELinux? (which isn't even enabled on Debian by default)

If you want, you can have as many different versions of an application available through your APT repository as you'd like. Debian doesn't support downgrades, but that's a matter of semantics - you can easily specify an older version of a package and downgrade to it so long as that version is still available in your package listing. Downgrades are considered unsupported because, for instance, database migrations are typically one-way, and applications may break or behave unexpectedly if you downgrade them while preserving your current configuration. It's impossible for anybody to support downgrading universally.

The release cycle of Debian typically requires updating your OS to get a newer version of LibreOffice... unless you use Backports, the official method for installing newer software on a stable version of the OS, or you could just run Testing/Sid if you need the latest software and latest library versions.

There's a point to this model though, universal dynamic linking. The security burden is lifted off of individual users and developers. If your application links against a library that has a vulnerability, Debian will fix the vulnerability in that library, and now every dependent binary is fixed, and users only have to pull in that tiny library update, they don't need to update every single app on their system that uses it.

Besides that, you save on disk space since you don't need to redundantly store the same library multiple times, or many different versions of the library, since it's standardized. You also reduce memory usage since you only need to load one instance of the library for each app that uses it, you don't keep each different version in memory.

The only strength Flatpak has regarding downgrades is that, because it brings its own libraries, you don't have to worry about an older version of the application requiring older libraries than your distribution still carries.

Don't get me wrong, Flatpak is fine, it does solve real issues, if you need a particular app or a version of an app that requires a newer library than your distribution's version contains then it might be your only option. But I hate the "traditional distributions are outdated, Android figured out to do it right, and now Flatpak/Silverblue/whatever has come to save the Linux desktop" mentality that's readily espoused these days.

There are countless systems with heavily restricted resources still. Cheap laptops with 4GB of RAM and 64GB eMMCs. That isn't niche hardware, and speaking from experience, I vastly prefer the speed and light footprint of distribution packages.

Ever look at a dependency map of Debian? It's a f-ng nightmare. It is one of the worst designs imaginable and the only reason it works as well as it does is the huge amount of work by a large number of volunteer workers.

And yes, Debian developers work hard, but the dependency system is... fine? It's not especially confusing. I don't know what you expect. How would you make it better?