One coud look at things like this...
Emery is serving up the same as what Wenger was latterly serving up (potentially not getting the top 4 and losing a Europa semi-final). So why change?
Past performance predicts future performance. So, under Wenger, we were always going to get more of the same and probably on a slightly downward trajectory, if you extrapolate league positioning.
If we continue to get the same from Emery over the next 2-3 years, I completely agree he should go and we move onto the next one.
I dare say we'll find out what we already know... it's Stan that is the real problem.