I think you're probably partially right on this one. Most wikipedia articles are fairly shallow compared to treaties dedicated to the topic. That's not a bad thing, as I usually use wikipedia articles as an introduction to a topic I'm not familiar with at all.

However, that means for those topics, people will have to start learning more in-depth things to contribute, or the experts will have to start contributing.

I don't think there's a limit to human knowledge. It might just be an indication that there's nothing new going on recently. Once a new field or topic opens up, I think we should see another explosion of edits.

In the meantime, it'd be interesting to see the edit rates for the other wikifactions, like wikitextbooks, wikiquotes, wikitionary, etc. Maybe the activities have just moved over there.

If this talk on lexicography is any indication, there's still a lot of work to be done in other knowledge domains. http://www.ted.com/index.php/talks/view/id/161

One of the points of the talk was that dictionaries need to be updated in form--it's been the same since Queen Victoria I. http://www.confusingwords.com/ was a related website that I stumbled upon soon after. It'd be interesting if wikitionary would be able to incorporate not just meanings of words, but new connections between words, besides just being related in meaning.