It would if they then syndicated the posts to publications around the world. German newspapers and magazines, for example, could have their pick of prominent writers they otherwise would not have access to and couldn't afford.
No sharing. Prominent people's opinions in Medium articles would make news and drive people to Medium content. "Colin Powell calls Trumps immigration policy misguided". That's a global headline. Traffic rules.
Everyone tries to think like a VC. Medium will never be the biggest money. What they can do is be the next '60 Minutes', not the biggest, but prestigous and elevating.
Having Powells weekly column is more valuable and provides something you can plan around and build a business model around. You can't do that with social media snippets.
You seem to be equating prominent writers with more revenue. But the real question is how do you get enough revenue even with prominent writers. It just' doesn't scale.
No, its not the quality of the content which makes it a business, this is exactly the misunderstanding of the whole mission to save journalism. Its never been a business to begin with.