[doubled112]

Because many of the changes are large enough that it will break, and that's expected.

The KISS principle applies here.

If a config file format changes in a service between version 3 and version 4, should the package manager be responsible for it? Or the admin?

Sometimes it's not just merging changes in.

In a non-rolling release distribution, you only need to worry about those changes during major upgrades. In a rolling release distro, they can change at any time. It's no different than a user reading the release notes for Debian 11 while upgrading from 10, except the upgrades are constant.