Not a change in Wikimedia, but a change in society
I think that the Coca-Cola example is apt. Are there better examples of what I think might be happening? For example look at the shift in TV comedies between the early 50s and the late 50s as the number of televisions grew in the United States. Early on, television was dominated by comedians from New York City, with New York sensibilities and experience in the Borscht Belt. That changed not because the nature of society changed but because the demographics of the watchers changed. The first television were sold and stations set up were sold and set up in cities, particular New York with more urban sensibilities. As the number of televisions expanded, more rural people started buying TVs, and the percentage made up of New Yorkers and urbanites shrank. The audience was no longer as dominated by the urban. You had a shift from Sid Ceasar to Johnny Carson.
Is the same thing happening to Wikipedia?
Perhaps Wikipedia has reached nearly its maximum market size in the United States and other Western countries. Perhaps there is something particularly American about Wikipedia with its openness and lack of hierarchy. An Asian society Wikipeida ain't. Just look at where Wikipedia is based. Is there any more American place that San Francisco and Silicon Valley? Ameicans have always volunteered more and donated more than others. As Wikipedia expands into different markets, demographics change. People in other places do not volunteer as much but off-line and on. Like changing comedy shows maybe the audience has changed and so there are fewer of the types of people who are attracted to editing.
Or maybe it is like the menu of McDonalds. McDonalds might be a world-wide phenomena but in each country it has adapted itself. It is not the same McDonalds in Hong Kong, Vienna, New York and Smallville. Is some places McDonalds sells lattes, salads and milk shakes, but in others it is fried chicken, rice and spaghetti. In American McDonalds, you clear your own table; nearly everyplace else people are paid to do that.
Perhaps the early adopters, those most likely to be persistent editors, have already been captured by Wikipedia. Perhaps all the low hanging fruit -- uber-volunteering American early adopters -- have been picked.
Bad news from the real world about "market saturation" with Coca-Cola: http://www.reuters.com/article/2011/02/09/us-cocacola-idUSTRE7180OQ20110209
Another point. Are we talking about readers or editors here?