Who says we need to fill it up in a reasonable time frame? If each disk costs a comparable amount to a dvd (cents) then the expensive part is the electricity (and maybe the equipment to write to them) and it's ok if most of the theoretical storage capacity goes unused. The important metric here for something like data backup is cost/byte to write and cost/byte/month to maintain.
No, I'm suggesting that it could take a decade and still given the numbers in the thread I replied to (which I agree appear not to reflect reality) be a viable product. It never needs to be 'full'.
