Bill Polian was an awesome GM. It went south when he put his kid in charge of drafting and player personnel. Grigson is a joke and the only reason he has his job is because of the huge talent that dropped in his lap in the 2012 draft (Luck, Hilton, Allen, Fleener). As far as coaches, not sure who Irsay could hire at this point but I'm sure it would be another disappointment.
Back to the Bills, after watching football for nearly 40 years, I have come to one conclusion - never trust a Ryan coached team. They're generally great coordinators (except the guy in New Orleans) but suck at being head coaches. I remember Saints fans bragging and making all these ridiculous memes when Rob Ryan became their DC - I just shook my head because I knew they were making fools of themselves.
Here is what will happen - the Bills defense will be very, very good and their offense will initially show flashes. Rex will same something fucking stupid like "Our QB is Tranchise Taylor!" Taylor will regress, defenses will pounce on him, and Ryan will continue backing him until it inevitably costs him his job.