One could also argue he had the luxury of the greatest QB in football history.
Bottom line for me is that no one, no matter how capable, can do both jobs well, given the complexities of modern football. And Dinwiddie is also play caller on offence.
I'm not saying it won't happen, but IMO you need a dedicated head coach and a dedicated personnel manager. The Argos have managed just fine to this point with a GM who does not fill the traditional GM role, but he has been surrounded by capable personnel people. Again, two of those guys are now gone, and who knows if Murphy is or isn't still around. I get why Dinwiddie might like to be GM, but if he thinks he could manage that role and the head coach role at the same time, I think he's mistaken.