All generalizations are wrong about individuals, but on the scale of 'Medium members' only the generalities matter, and my primary assumption is that, on average, the socio-psychology of Medium members reflects that of society. None of my assumptions say anything about individuals.

Yes, all sorts of people will upgrade to Friend membership, for all sorts of reasons. But this isn't about the Friends themselves, it's about Medium's choices and what they reveal about Medium's aims.