I like the idea, but I think most users never would/could author their own UI's, and many who could would only want to a small percentage of the time.... but maybe your idea could work if there was essentially an API marketplace AND a UI marketplace, and services competed on both of them seperately... v1 from a service would (probably) always offer both to be usable but if it published some contract for how they talk to each other so they were decoupled and anyone could author a new UI (and, I suppose if it were a marketplace, sell it, but maybe given constraints only selling the API is realistic and UI is more FOSS/enthusiast driven/fork based?) and make it discoverable as an alternative to try out.