We allow multiple files download, we don't limit users, that's why when a user sees that he has option to get his file using our service he does that. We are here to make the end user happy.
We calculated exactly how much we can pay to our users and still remain profitable, we don't give crazy and not realistic rates as some do for a short time before they close shop, we are here to stay for the log run.
About file retention, that's not an option to keep more then a month files that weren't downloaded even once, it's not going to change if we wish to keep up with this speeds!
Good luck to all :D