Good online communities die primarily by refusing to defend themselves.
Somewhere in the vastness of the Internet, it is happening even now. It was once a well-kept garden of intelligent discussion, where knowledgeable and interested folk came, attracted by the high quality of speech they saw ongoing. But into this garden comes a fool, and the level of discussion drops a little—or more than a little, if the fool is very prolific in their posting. (It is worse if the fool is just articulate enough that the former inhabitants of the garden feel obliged to respond, and correct misapprehensions—for then the fool dominates conversations.)
So the garden is tainted now, and it is less fun to play in; the old inhabitants, already invested there, will stay, but they are that much less likely to attract new blood. Or if there are new members, their quality also has gone down.
Then another fool joins, and the two fools begin talking to each other, and at that point some of the old members, those with the highest standards and the best opportunities elsewhere, leave...
This about the Internet: Anyone can walk in. And anyone can walk out. And so an online community must stay fun to stay alive. Waiting until the last resort of absolute, blatent, undeniable egregiousness—waiting as long as a police officer would wait to open fire—indulging your conscience and the virtues you learned in walled fortresses, waiting until you can be certain you are in the right, and fear no questioning looks—is waiting far too late.