Periodically saving offline backup files does not contradict with also running multiple online devices at the same time.
You can access content in ways other than by restoring a backup to a Delta Chat client.
I will still retain my authorship and copyright to all messages typed in by myself even if the given group has disappearing messages enabled. Hence, I have all rights to back up and even publish my messages as I see fit. Almost all groups I participate in has such a setting in place, so I would estimate that 99% of my messages have already been deleted by now. See here for a simple “oneliner” example you can run from cron to achieve this (I also index it and produce nice HTML from it)