I have a "lurker-to-subscriber" hypothesis.

It seems Mr. John had enough loyal readers to put him into the $100 to $1000 a month category of Medium income.

But to get above that threshold, he would need to show Medium that his material could move lurkers to paying subscribers. If all he has is loyal readers, no new money comes into Medium. Hence no higher frequency in the feeds.

Writing more articles would not be enough if the lurker conversations are not there.