I guess we disagree about the point of backups then.

We just use different threat models.)

For me, the main threat is disk failure, so I want to get new disk, restore system from backup and continue as if nothing happened.

Surely, if your hardware or OS configuration changes, you should not backup /usr, /etc and other folders.

However, the proposed workflow could be adapted to both scenarios: a single snapborg config backs up snapshots from a single subvolume, so I, actually, use two configs: one for /home excluding /home/.home_unbacked and another one for / excluding /var and some other directories. This two configs have different backup schedule and different retention policies, so in case of hardware/OS change, I'll just restore only /home backup without restoring /.