Although I couldn't see a way to do it when I first looked at Medium, I'm told that there is now a way to use it with your own URLs.

I haven't bothered to examine how it works, but if you can do that and you have a disaster recovery plan that replicates your entire output to a different service (e.g. GitHub Pages) while retaining the link structure, then perhaps Medium is now superior to Posterous's original offering.