You're strawmanning my argument. I did not say, as you imply, that active user growth should continue indefinitely.
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.