It just takes one storage service to decide to bridge the gap and a wonderful era of innovation can begin.
This isn't a question. In 2016, the technology is mature, we know how it works.
Here's a sketch of how the service would work.
That's it. Now I can hook my JS-in-the-browser app to your service. The user manages it through the UI you already support. And we've opened up a new area for developers to be creative. And most important, it says the exploration of great writing tools can advance outside of Medium. (That's how important Medium has been for the last few years.)
BTW, for Amazon, they would use the S3 API, which is supported everywhere. The apps would pop up very quickly for their service.
It's a total logjam and could be broken by one storage service deciding to help the users break free of silos.