Unfortunately Dropbox's revision history isn't reliable enough for a backup. If you notice, their marketing carefully avoids use of the word "backup" despite the features seemingly implying it.

I had a client where a glitch in the Dropbox sync caused 300k+ files to be deleted when we added a new PC. Dropbox support was unable to successfully undo all the file changes, and I had to get a ticket with CS escalated to a special team to get everything restored. Even when it was finished, they could not give a guarantee that every single file deleted was restored.

Maybe things got better since they've launched Dropbox Rewind, but given that revision history has been a feature for years and it still didn't work right, I no longer trust them as a backup.

I'm completely soured on dropbox since they dialed the greed up with the arbitrary device count limit and nagging suggestions to upgrade that can't be turned off if you're near the max. I don't care enough to stop using it but I will never give them money after this.