Already been said it won't be him, but Macdonald would be foolish to even consider a college head coaching job at this point. He'll get an NFL head coaching offer soon if he keeps it up. May not be this year but it's coming.
So I sort of disagree to a certain extent. I think if you're a quality, younger offensive coordinator in the NFL, and your team can score points, you can pretty much write your ticket to getting a HC job within 2-3 years. There's 8 HC vacancies right now, and the only two defensive guys who got gigs were guys that were already a part of those teams (Pierce and Mayo). Three are offensive guys, and at least one more for sure will be, and possibly all of them.
Guys like Belichick and Vrabel are quite possibly not going to get HC offers in a league where 25% of the teams were replacing their coach. That sort of sends a message to teams about how they value defense in the NFL at the moment.
I don't think MM should go to college because I think whatever HC gig he could get in college is frankly a step down from being a DC in the NFL. He's not getting the Michigan job (nor would I expect them to offer it to him), and so he'd end up at a mid-tier D1 program in a Power 4 conference. Probably more $ for him, but an NFL DC job is a better gig.
There's plenty of coaches who were "rising stars" who never got HC offers and now seemingly have a long way to go to get them. Wink was a HC candidate like 3 years ago. Bienemy was the "sure thing" HC candidate and now he's about to maybe be an OC on his third team in three years, and he's an offensive guy. It's brutally hard to be a DC in this league and get serious consideration for HC jobs right now. And that's actually made worse by how long your team lasts in the playoffs. Teams don't want to wait 3-4 weeks or longer to hire HC's.