The proven guy in your scenario is 50 years old and has went backwards in his career. The "green guy" in your scenario coached a WR to single season numbers that would be #1 on our all time list.
I don't hate the coach from SMU that coached a WR to over 100 receptions and 15 touchdowns just because he is "green"
So if the new OC who put up numbers wants to bring his WR coach who had success, why would I complain?