Normally, I really wouldn't care if Medium holds the copyright of my articles...
...BUT I do care that they may have the opportunity to sue me for referencing my own work if I publish elsewhere. What's more, this allows for the possibility that when I write elsewhere, I might inadvertently use words and phrasing close enough to my articles here that Medium could sue me for violation of the copyright.
To the movers and shakers of the Medium community, THAT is wrong.
I'm not going to leave Medium in the foreseeable future. I'm making a bit of money here and there and it certainly helps. I'm sincerely grateful for how Medium has enabled me to improve my writing and expand my audience.
But please rewrite your terms of service in such a way that we know we won't be sued for inadvertently plagiarizing our own work. Besides, the moment this happens, the one being sued would almost certainly raise a public stink about it and would cost you a *lot* of membership. In other words, if you sue an author for compensation for copyright violation for copying his or her OWN work, you're going to wind up costing yourselves far more than any compensation the courts may render.
Beware of greed. Remember, Karma does not play fair.