Very cool idea. Actually I believe that there should be some cooperation between centralized traditional exchanges and decentralized crypto exchanges. For example it's probably impossible to do HFT on blockchain (high frequency trading). But if you have some central access point in you can trade through it, and the distributed exchange is used for clearance (it's very sketchy but I hope you get the general idea)
So there can appear some centralized platform giving access to all decentralized exchanges, like a tratidional broker provides access to various trading networks.