This is a classic lesson that everyone learns called Sync Is Not Backup. Everyone learns it eventually and then writes a blog post on it. It used to be a classic HN meme:
Well, and that a sync can destroy existing files while a proper backup flow can't.
If you had 3 syncs in 3 different providers but deleting your local folder causes all 3 copies to be deleted, that's still no good. You need another place where files can only be added to and never (automatically) removed.
I wasn't referring to syncs at all, but actual point in time backups, so I agree.
There are syncs that can also be configured to not to delete. It's how syncs can do version control.
If you use an off the shell backup or NAS like a Synology or QNAP it will take care of all of that for you. You can plug in external drives to it for copies, an then also have the NAS ship off backup copies to multiple places.
Since the cloud is someone else's computer, it's good to have your own copy of it too on your own hardware.
Yours is an extremely important clarification too.
Syncs get even worse, because most promised unlimited backups of revisions (thereby implying restoration)... And now maye when "deleted" files take up too much room, they disappear.
There are file clouds out there that seem to be better for the natural kind of use, whether it's backblaze, tarsnap, sync.com, self-hosted options and others.
It's essentially the same story as RAID Is Not A Backup.
Like with sync you have redundancy, but Lord Redundancy never said he was also also Count Time.