Serious suggestion that I haven't researched myself:
What if Medium charged all users/readers one dollar per month? Then, it took that revenue, kept 50% of it, and divided the rest amongst its content creators?
Sure, it wouldn’t be enough money to fully fund Medium, but it would be added revenue and it would incent content creators to use the platform.
(My understanding is that YouTube does something similar.)
Plus, charging users 1 dollar per month is probably enough to keep out the riff-raff and avoid a potential Twitter situation, where abuse and spam is a constant problem.