Why not use a system similar to STORJ for distributed hosting of video files? This would distribute the load while also solving any redundancy issues. Uploaders could contract with storage nodes on chain for the space and bandwidth usage. Give uploaders control over the level of redundancy as well, which would reflect in the cost of the contract. You could have both short term contracts as well as long term archive type contracts or whatever you want to call them. I think it would be a bad idea to limit something like that to steem witnesses though... it would be a bottleneck for one, but it also wouldn't be very pro-decentralization if you catch my meaning. I've got a large file server on a fiber line just sitting idle waiting for the right decentralized storage network to come along. Storage is cheap... if you create the right inventive people will support it.