>file sync is backup against the things most people have to deal with

I would say accidentally deleting a file is more common than a house fire, so NO, file-sync is not a backup nor is RAID (block-sync) or clustering (data/state-sync).

A backup is/should be a non-modifiable snapshot of data/files maybe even a state.

I give you an example:

You have a password-safe (as a file). Your computer blows up, but you are safe, because your password-safe is synced on Dropbox....but oh my god what was the password to Dropbox??? Your restore is gone because you forgot to "backup" your password-safe.

However you are not a Loser, being a Admin is a ~lost art and a really good paid one (again).... ;)