Tony's numbers at Medium Day and now Buster's update prove it. The number of subscribers is increasing fast since Boost started. Stickiness must also be seeing encouraging numbers otherwise churn would offset new users into negative stats. I can also corroborate that from a personal stats perspective, I am gaining subscribers (I don't mean followers) instead of losing and out of the referred members I gained in less than 2 years, 80-90% are still here. The reality of the matter is that Medium is not a service that people NEED in their life, it's something they WANT, therefore people paying for it must come with value, and when people keep paying for it, it means they're happy to pay it, otherwise they would just stop and leave.

I can only suspect that Medium has a ton more stats on customer sentiment, data you or I have no access to.