Storage is getting so cheap so quickly that this is a disappearing problem. Also, glossing over the barriers to implementation, your solution to the stated issue makes the overall fidelity of files on the network much less predictable, particularly for those with fewer up-votes. To compensate, the minimum number of copies of each part of each file that the network would have to cache to be reasonably certain of its ability to completely deliver to new downloaders the files riding the minimum up-vote threshold would have to go up. This would in turn push the minimum up-vote threshold up and make it more difficult for obscure content to survive on the network.