very well put

You make excellent points.

Absolutely - that so much of Reddit's niche and success was being a place where people already were. Folks who made memes or wrote articles went to the place where the audience for that content was pre-built and was focused in a predictable way. Folks who had questions or contributions to make went to the largest community they could find, tied to the content they were focused on.

Absolute reader numbers or absolute activity are only indirect metrics, what the community needs is a large-enough dedicated core to keep a sense of culture and continuity alive, a steady flow of new content or topics, and enough incoming members to replace natural attrition. I find that the last two tend to be strongly linked - for a niche-topic community, one of the best sources of content and activity is beginner questions. Experts often don't have a ton to talk about day-to-day, unless some big news or development has happened, in which case the topic is explored until exhausted and then dropped. But have a steady flow of newbies there to ask the experts questions, and that will prompt not just responses for the newbies, but conversations among the experts on the side.

Yeah, I don't think there's some obvious number we can use to quantify the success of the Fediverse. It's more of a feeling. How often do threads feel like they have good discussion? How many niche communities are available to you?

Past a certain point, more comments in a single thread doesn't do much. You'd almost never read all the comments in a front page r/AskReddit post, for example. That's too many comments on the same topic and past a certain volume, quality comments can't rise to the top anymore, anyway. But there's so many niche communities that don't have enough people here yet to take off. Especially local ones.