I completely concur with your vision.
I’ve just killed a project that attempted to migrate users off of a Facebook Group.
I failed.
Despite offering them a superior set of Discourse functionality, including bespoke plugins I had written to provide really rich domain functionality that blew a hole in Zucks side the one feature I could not compete with was the ability to show an aggregation of all the interest groups they were subscribed to.
“We loved your website, Robert, but it was just more convenient to scroll down Facebook” 
If we had a single web page and web app that could show a feed of all your many Discourse site accounts now that would be just great.
Perhaps an RSS reader would suffice. Has anyone tried this? But ultimately you need a way to access with your individual accounts in the way the (excellent) iOS app does.