The site will simply not operate with large margins like other sites. The cost of keeping the servers and development running will be covered by a premium plan that's targeted to heavy users.
I don't mean to be negative, it's just that I've seen the transition from free to freemium to ad-supported to Rapidshare to dead happen over and over again. These services don't seem to operate with large margins - from the outside, it looks more like all of them are on the brink of bankruptcy.
Lets imagine our average citizen uploads quarter of a gigabyte, that means 4,000 visitors uploading files per $10 client. Assuming you can keep other costs really low, it's not too outrageously expensive.
Rapidshare was a service that had to swiftly and reluctantly change their business model. They were operating out of large offices with over 60 employees and due to the change they saw a sudden drstic drop in revenue.
Rapidshare's financial information isn't public since it's a private company, but one could argue whether they were actually losing money or not making the profits that they expected at the time of the shutdown.
Then came the FBI raid against various filehosts. I remember few of the other hosts got taken down and charged, while others shut their site down as precaution. I remember Rapidshare starting their decline during this phase, something tells me Rapidshare did it on purpose and 'ran away' with the earned money before the feds got them.
Well thats my theory :)