Earlier quoted context omitted.
It's impressive that it's stable considering there's probably a lot less to do on Wikipedia today than there was in 2007.
In what sense is there a certain "amount of work to do" defined independently of the number of volunteers and their interests? The only thing I can think of is that as the wiki grows in size, the amount of work that needs to be done to maintain fixed quality (combating vandalism, counteracting normal decay, fixing link rot) goes up roughly proportional to the number and length of articles. I guess you mean something…
But you make a good point that the maintenance work probably grows linearly with the size of the corpus, so really the flat number of active editors shows that the number of editors actively engaged in the research and writing of entries has probably actually declined significantly since 2007.