Find this perspective a bit short sided. If Riley had been hired at the time of Callahan, you'd be saying Riley is who destroyed our traditions. That is focusing the argument essentially on timing. If we focus entirely on the field, Riley is without question the worst (as you stated).
I can't believe I'm defending Callahan but all he did was bring the system he believed in at the instruction of Pederson. You could also argue Callahan's system worked.
Now if you want to argue who is the worse AD, timing must be apart of it and I personally would put Pederson over Eichorst.