The repository system on Debian, Arch, Fedora is clearly not a walled garden like Apple or Microsoft's app stores. Any site can host a repository that you can add to your package manager's sources, and you're free to install .deb/.rpm packages from any source you want, not to mention tarballs or compiling from source code. Also, there's no motive for lock-in like mainstream app stores have.

Only if those sources keep things up to date with the changes in the main repository, and every other repository the user uses, so as to avoid conflicts.

> not to mention tarballs or compiling from source code

Seriously? You may as well say iOS is open because you can root it.

The reason for the conflicts are rarely if ever at the package layer, and more often than not with upstream changing dependencies on a .x update.

This leaves the distro maintainers two choices: either recompile/restructure the whole distro to placate one upstream, or freeze said upsteam on an older version and suffer their social media wrath.

Or because it's a new feature release instead of just bugfixes. At least in e.g. Ubuntu, SRUs aren't accepted unless they're security or bug fixes, which makes it impossible to get newer features back to stable releases. Snaps provide a way out of this by removing the ability to break other applications with an update (since there are no dependencies).