Disclaimer: this is NOT a proposal to change the planned model of issuance of Safecoin. This is a merely theoretical discussion that may serve to fork the network or just for the sake of discussing.
I wondered whether the main problem to switch from pay-once-store-forever to pay-per-time is network security (time stamping). Probably it is, since on SAFE as it is designed currently there is no relation between me and my files but a key that I own and that allows me to access my files. In that case I see that a Pay per Time proposal doesn´t make a lot of sense. Still I see some merit to think about it if there is some solution to make it possible while delivering the same secure access.
Imagine that unlike now there was a set time in which all SAFEcoin are distributed. Imagine that time would be 4000 days (~11 years) and distribution would pay out in a curve that reaches maximum distribution after 5 years and eventually slows down until all coins are distributed. These coins serve as “subvention” of farmers cost.
To put content on the network, you load money to an escrow address from which money is constantly discounted. The content is chunked and distributed. Farmers are concurring to host chunks because this is what gives them money. Farmers clients can adjust the minumum payment so the price is eventually sorted out between those who can offer the most efficient farm. At idealistic farming levels, the price per GB drops close to zero - if there are only commercial farmers it will be slightly higher than cost per hardware + electricity and bandwidth.
Unlike the current design of Safecoin this sets the owner under pressure to sustain the content on the network by loading money to the prepaid address - so this is a very different economical approach. However, since this can be done with a public prepaid account it doesn´t have to be done by the owner alone: Imagine some whistleblower uploaded material onto the network and the messenger gets killed, other people could provide the account with coins to prevent the files from expiring.
I also see another merit: Let´s say I don´t want to pay anything for hosting my files anonymously over the network I could simply upload 10 GB of files and host 40 GB. This way I would always earn what I pay, but have my files stored securely over the network. In case I have to shut down my machines for some reasons over some time, I disconnect my devices and load some extra coins to the prepaid address. Let´s say I moved somewhere and had to drop out 1 week, then I will simply host 80GB for one week after that and I am back again at about zero (if exchange rates weren´t massively volatile - if you counter-host your stored data directly you don´t have that effect).
The advantage over the current concept of SAFE is that the network cannot “collapse” - however, if the price isn´t stable (which may be the case, particularly at the beginning) it may lead to unpredicted discounting (or hoarding) of prepaid coins. In a sense that is inevitable, but I think if prices are highly unstable this would destabilize the current model as well. Also, it´s possible to hedge against that, allowing for more network stability. Anyway, the economy wouldn´t depend on exponential reduction of storage cost.
The main advantage I see is that you can run the network at 0 cost, once you decide to quadruple your hosted share.