> the chances of something physically happening to your local backup (the hardware fails, someone knocks it off the shelf, it gets lost in a move, there's a fire) is vastly greater than the chance that your online provider loses your data.

This is the key part: almost all of the failure modes affecting local storage also endanger local backups. Cloud backups are intrinsically less risky in that regard, although a local provider might still be affected by a massive natural disaster.

The remaining class of errors, however, are nasty ones like bit rot, human error or malice – all of which are tricky to prevent.