That's actually not the whole point of a backup in this context. It's important to remember that the Backblaze Personal Backup service is essentially an emergency extra copy, not a data archive. Once the client backs up a file (or more accurately a revision of a file), it checks every 30 days that the file still exists locally. If it does not, then it will be deleted from the cloud backup.

For example if you plug in an external harddrive and back it up with Backblaze, if you don't plug it in at least once every 30 days, then its cloud copy is deleted. So no, the intention of Backblaze is not to keep your offline machine from several years ago securely backed up.

That being said, you can pay for unlimited revisions which just stores your data in b2 and you pay per month on the extra storage. In that case then yes I would expect the data from years ago to still be present. But that's a recent addition, and not what the OP was using.