You hit a sensitive issue.

What is Medium's purpose? Its founder and current operators see it as a platform for aspiring writers to see their names as authors on the internet. If Medium sticks to this concept, it might never reach five million members.

If Medium focuses on attracting and nourishing authors from outside the mainstream media, it can offer professional quality writing that complements what people can find in major media outlets. Also, its open format could allow publishing for people with something interesting to say, a little talent, and some willingness to work on it.

The focus would be on bringing up material worth reading. Medium can expand significantly only by offering readers a better experience than others.