How do you expect it to continue growing? There aren't enough people to continue active user growth. It's like your expecting it to be a pyramid scheme that makes money as long as you sign up more people than exist on the planet.
Take a look at this graph: https://www.gwern.net/images/2012-henkvd-activewikipedians.p...
First, one would typically expect the number of active users to keep increasing but for the rate of increase to gradually decline, not for the number of active users to actually start decreasing gradually after multiple years of very rapid increase.
Second, if you don't see something wrong with that graph, get your eyes checked out.