Do you know if FileCoin is proof of storage? I thought it was POW derived from Bitcoin. At least that was what they were planning a year ago.

And you are right about Sia and Storj. As such, they could have made use of some established blockchain/coin. Maybe they still needed their own chain in order to be independent and free to develop the chain to suit the case. It will be interesting to see how MaidSafe proof of storage works - that is, if they ever get it out.