Your smartphone is already an always connected device, and you can manually do periodical backups on external drives which is a good practice in any case.
You can also set which device rules the file modification (sent or receive only as a folder).
I think it’s ok for most of the people. If you have a tons of precious files to save I can agree with you, but I think this type of “off-grid self-hosting” is just the future of personal data management.