Humans don't know how to scale, and parasitic extraction forces are always looking for a new host body, but the core problem IMO is that there is no shared and agreed upon "point", "vision", "ideal", "value", regarding the "purpose" of any "community" (whether that is a forum or a company or a product or ...), so no one can tell what's actually good about it, ensure that newcomers perpetuate and/or magnify that, and recognize when we're drifting off course.
It's a qualitative difference, but unless one can articulate and recognize and enforce and etc the core quality/qualities, it breaks down over time. People who don't understand or care why a thing was "good" are simply not going to perpetuate or magnify that "goodness" over time.
And, of course, even the "core who were there when it mattered" are highly unlikely to agree on what made it worthwhile. Ask me why "Metallica before they sold out" is better than "Metallica after they sold out", and you'll almost certainly get a different answer from 100 other people who feel that very same way. Etc.
It's a qualitative difference, but unless one can articulate and recognize and enforce and etc the core quality/qualities, it breaks down over time. People who don't understand or care why a thing was "good" are simply not going to perpetuate or magnify that "goodness" over time.
And, of course, even the "core who were there when it mattered" are highly unlikely to agree on what made it worthwhile. Ask me why "Metallica before they sold out" is better than "Metallica after they sold out", and you'll almost certainly get a different answer from 100 other people who feel that very same way. Etc.