It's based on the assumption that storage costs per GB will continue to decrease for their definition of "perpetuity." If you assume a consistent fall in costs,

It's based on the assumption that storage costs per GB will continue to decrease for their definition of "perpetuity." If you assume a consistent fall in costs, the total cost becomes a convergent series [1] and there is an upper limit even with an infinite number of years. Suffice it to say, physics is probably going to have a lot to say about that assumption in the coming decades. [1] https://en.wikipedia.org/wiki/Convergent_series https://en.wikipedia.org/wiki/Convergent_series