Most people actually would have no idea where to start to build their own back up server. So, for your average user, Backblaze type of online backup is the only real back up option.

Sure they do: buy an Apple Time Capsule. Or similar appliance that contains two drives at RAID 1. Think backup appliance, not Debian box.

And exactly — the average person would have one Backblaze account per device, which would get … pricy quickly.