

Malware targeting individuals rather than servers do not need privilege escalation. They just need to run as the user and swipe cookies/credentials/wallets etc. Privilege escalation would allow them to do catastrophic damage but that’s not the point in that case.
That doesn’t address the original point which is whatever’s shared has to exist on all machines.
Either way, you would need to backup your data if you were self hosting Nextcloud or friends so you do need multiple copies of it anyway.