Aren't one-off payments to cover running costs provably unsustainable? It relies on continuous growth to support old users. I personally would want to rely on a service that might close at any minute because it stopped growing.
Combine that with underlying costs (storage, data transfer, computing power) which are trending downward and there’s no reason it shouldn’t work.
Say they raise enough to earn interest on their capital.
Say they make 1% interest per year.
Storing a GB costs $10. 1% of that is 10c.
How much does it cost to store a GB for a year?
Amazon charge ~1c/m for infrequent access and 0.4c/m for Glacier.
So somewhere between 12c and 5c a year.