This means for "unlimited" they can recoup 6x the storage i was using per year through my fees. So the question becomes why it's not sustainable? Too large a company and not sustainable due to employee salaries? Not economical enough storage prices (ie using enterprise SAS disks rather than cheap SATA)? I'm guessing since they buy large quantities of disks, they could get drives for even cheaper than what you get on NewEgg.
This is why I believe that this model _is_ sustainable, assuming that it's done right. Also why I switched from Mozy to Backblaze because I felt that Mozy was gouging me by taking away their unlimited plan and replacing it with a tiered plan.