Medium, like YouTube, is a platform for creators to make money with little friction while also providing exposure that is difficult to build on their own. That, above all, is why platforms like this are successful.
That doesn't change that the "idea" behind Medium is good and that there's a market for a blogging platform that gives authors easy access to a large audience.
Alternate possibility: the core idea is a good one, but they raised too much money. Now they have to live up to an impossibly-high valuation and can't just be a blogging platform.
I'm reminded of the dot com bubble in the late 90s.