What particular factors might have begun to inhibit participation in 2006, when we know it began to stagnate?

Yeah, I don't think we'd want to make a specific conclusion like "articles are already created, and people ran out of things to contribute". But we could probably make a broad conclusion that there is a natural slowdown, and that it's not based on idiosyncrasies like the introduction of flagged revisions, or changing the privileges for anonymous editors. There are things that happen as a mass of people develops into a community with a culture, and we need to create a better on-ramp for new users so they can mingle and embrace that culture.