When you do this, you work with the platform as long as you want. Then you can walk away from it - all your content is in your git repo and up on your web host as though the middle layer never existed. Another service can pick up on the same task, which you then point to the same endpoints to continue. If you know how to use git and your host directly, great. If you don't, who cares, these services take care of it for you as a transparent endpoint.
In fact you could even charge money for this kind of thing. Or even make it more open ended than just blogs and web pages, but that's where my thinking got jumbled on this idea and I only got as far as buying a domain for it.
Obviously sites like medium and everyone else would never do this, because it means you can leave their platform immediately. People who are capable of executing this idea pretty much don't need it, and it's otherwise hard to monetize. The desire to lock people into things is really what ruins everything.