If you find that significantly expanding the retention limit doesn't fit within the current pricing structure (as I expect it wouldn't given how competitively priced Backblaze is), consider offering it as an add-on feature, as Dropbox does with Extended Version History (+$40/yr for 1-year retention of deleted files).
I think at a high level users might be OK with paying for their actual stored file size.
A way of improving the storage size might be to allow the user to white-list (with some defaults) folders more likely to have 'third party' files (E.G. C:\\Windows, C:\\Program*, /usr/) either excluded if they're on a list of common files or de-duplicated with a public list of common files (and check-sums). It would be useful to add-on programs and scripts if that list were public, and if a way of cloning those files in to a discounted pool were possible by 'uploading' them again (to ensure the customer actually /has/ that file and thus presumably the right to restore it).
