Why not just do this without the overhead of mining and maintaining a blockchain? Surely there's a more efficient way of maintaining a public distributed file system.
If you have a system where anyone can provide storage and anyone can buy storage, then you need to have a system to verify that the files are actually being stored. As I understand it, that's filecoin mining.
Filecoin eliminates a lot of the overhead of selling storage, but it adds some overhead all its own. The big question is: which is larger, and by how much?