What does this mean? How do you store an arbitrarily long sequence of changes on a medium of fixed size without overwriting? Eventually you will run out of disk and old data will have to be overwritten, which might have contained the only good copy of the corrupted file.

> If a new backup causes corruption to back-propogate then it's not a backup

I'll go further and say that even backup that forward-propagates corruption is not backup either - all the incremental backups from the moment of corruption are worthless. Bottomline: if your backup cannot be restored with integrity intact - it's not backup!

To paraphrase:

>>>> If your data gets silently corrupted and you keep backing up that corrupted data, eventually there won't be any backups left that have the original uncorrupted data.

>>> You shouldn't delete old backups!

>> Storage is too expensive to keep old backups forever.

> Backblaze! ... will delete backups after 30 days.

Yes. So 30 days after your file was corrupted, you will only have corrupted copies left.