Once they opened up to everyone, they captured an entire generation. And once they introduced Chat, they basically killed every other IM service [1]. At this point, it was game over for everyone else.
Meanwhile, Medium is another Tumblr, or another Blogger, or another LiveJournal, but the visual design attracts thought-piece bloggers instead of personal ones. This is a self-selecting but heterogeneous group, so there's no strong network effects. How do they grow beyond this, or adapt to monetize their existing userbase? What makes them think they won't just jump to the next free blog host?
Many can also make the argument that facebook is worse than Myspace, quite validly for a number of reasons.
And making money from opposing models would act as an anchor, preventing from following the direction they believe.
[i]So, we are shifting our resources and attention to defining a new model for writers and creators to be rewarded, based on the value they’re creating for people. And toward building a transformational product for curious humans who want to get smarter about the world every day.[/i]
He wants to give great rewards to people who create great content. It's not just about views, numbers of followers, or how many people shared the content. It is about how this content impacts other people's lives and how positive it could be to have your brand or signature supporting the creator.
I personally think it's kind of hard to achieve this on a big scale. It is the opposite of the most successful program on the internet. Google AdWords.
But will change a lot of concepts if he succeed
The same (silly) logic was once applied to facebook. "It's just another friendster/myspace!" people said.