This is more true of communities built around ideas rather than activities, and there are also moderating tactics you can use to avoid it. For example, online communities based around astronomy, or radio-controlled airplanes, or baking, or other hobbies remain relatively healthy even decades after their founding.

Also there are communities around conventionally caustic topics (eg. Lambda: The Ultimate for programming language design, Penny Arcade for games) that manage to survive for a decade+ with no loss of quality because of moderation policies. For example, L:tU has an "avoiding ungrounded discussion" policy - every thread must be centered around discussion of a published academic paper, which first of all keeps the focus on people who actually do work, and second of all discourages everyone without the background to read and understand academic papers.