The hot name coordinator the year they hired Trestman was Bruce Arians, the reigning coach of the year, who went on to have enormous success with two different franchises.
Your point about offense could easily be reversed on defense, too. They've already been down the road of the gritty culture guy and that left them with John Fox and Matt Eberflus, who happen to be #2 and #3 on the all time worst record list in franchise history. As much as we all hated Nagy towards the end, he made the playoffs 2 out of 4 seasons as the head coach, which we would all kill for today.
The problem with the Bears is that they never go after the big fish. They chose not to go after Jim Harbaugh last year, and he's got the Chargers already in playoff competition. Ben Johnson is the biggest fish in the pond this coaching cycle, and he just so happens to coach the right side of the ball. For once, I'd love to see the Bears make the obvious hire and stop being afraid of paying big money for a coach in high demand, like Johnson.