it really is frustrating how little of the quality of a person [...] actually manages to rub off

Wait, you have a model which says it should?

You don't learn from a person merely by associating with them. And:

onto the legions of Internet followers of said person or cause.

I would bet a fair bit that this is the source of your frustration, right there: scale. You can learn from a person by directly interacting with them, and sometimes by interacting with people who learned from them. Beyond that, it seems to me that you get "dilution effects", kicking in as soon as you grow faster than some critical pace at which newcomers have enough time to acculturate and turn into teachers.

Communities of inquiry tend to be victims of their own success. The smarter communities recognize this, anticipate the consequences, and adjust their design around them.