I don't think Medium has fully grasped that people read most content because they are bored, not because they really care about the content; thus when you put the content behind a paywall, most people will just go find other ways to amuse their bored brains on the internet.
Content on the internet is usually so shallow it's bordering near worthless; lots of people just repeating other people over and over. When it comes to explaining a concept, most sites on the internet just cover the basics and promise they'll get to the deeper stuff later (and never do.)
The nice thing about the old book model (Before ebooks), is the amount of work it took to get a book published ended up filtering out all the mediocre content which we see today on the internet because it is so easy to hit publish somewhere. Paying for a book is worth it, as the book is actually a complete thought. The stuff people are trying to charge for on the internet is just mostly low quality stream of conscious writings with no actual content behind it.