I think community is being conflated with enterprise, in your examples. Appealing to larger audiences is of course different than filling a small niche. Communities proper aren't necessarily navigated at the helm to draw in more people, though some do (particularly if they aren't strictly communities).

Easy anecdote: internet forums. Many have since shriveled away. I can recall some from the past that were clearly at their most vibrant when the membership was high and new people were constantly joining. This all changed since the broader shift over to large social media sites. In most cases a group of cliquey seniors remain with low engagement.

One thing I noticed as these places shrink is that the conspiracy nut voices who spam the place get louder. Their activity level stays more or less the same. Once everyone else leaves it's just unbearable.