This is true, there is definitely a lifecycle to these sites (which even perhaps applies to forums too), it can be seen in all large online communities (and perhaps all communities?).

There's another even more fundamental problem though which emerges at every stage. The people who want to participate the most at any stage are not the most qualified, and the people who are qualified to answer are busy doing instead of answering. It's not just that inevitably you need to kick out bad content, it's that the people doing the kicking out don't know enough to judge what is good and what is not.

So by the time you get to the recruiting moderator stage most of the people interested in becoming gatekeepers/moderators are not at all the people you actually want to be gatekeepers (those people are generally too busy or not very good at such things), and your moderators are a self-selecting group who are more interested in form and adherence to rules than content.