Ironically I’m currently writing my MSc thesis in exactly this idea of a hybrid blockchain structure where public and private networks complement each other for added security and scalability. My only fear is that consortium networks could take the place of public networks (i.e. private networks connected to a consortium run network). Which isn’t too terrible but it still wouldn’t be the right way for blockchains to go IMO. In terms of hierarchy, I think the public networks always has to be at the top as the main ‘ledger’ with smaller purpose based and industry based networks stemming from it.
Either way, thank you for the piece it’s an interesting read :)