My first online communities were for a video game called Counter-Strike.
From circa '99 to around 04 it was the most popular online multiplayer first-person shooter game. Unfortunately, we didn't look at the broader trends. We didn't see this coming:
Our TFAS (total feasible audience size) declined and we were trapped in a collapsing bubble. This happens a lot. A community just shrinks inside it's own interest bubble.
By comparison, look at the trend for it's close cousin Call of Duty.
Call of Duty didn't create gamers, they attracted much of the same audience we had. Instead of evolving with the audience, we remained resolute to the interest and shrank as the members vanished.
This was a tragedy. After years spent building a community, fostering the connections, facilitating events/activities, we then failed to evolve with the community and watched those same people, with the same ties, move to new communities with a specific Call of Duty focus.
The lesson is this. The interest that unites the audience is flexible over time. It's easier to change the interest than the audience. Keep a close eye on not just the trends for your topics, but any other rapidly rising trends and incorporate them into your community's topic.
If your precious members are interesting in something new, you have to include that in your community (or be prepared to lose the members).