A few other comments were asking what Medium actually offers users, and I think you've nailed on the head: The potential for a massive audience if you really can write well, but maybe can't market well.
With that said, I've never had as much luck with native Medium traffic as much as on my first post, so I've since had to become much more clever at finding readers. Once you know how to properly distribute longform content, platform becomes less important.
I'll continue to use Medium though, because the design is incredible, the writing experience (but not the editing experience!) is a pleasure, and there's always a small, reliable amount of native traffic I'll get from Medium readers. And to me, the opportunity that a piece might go "Medium viral" is way more valuable than the downside of not being able to plaster my posts with popups.