He got fired by a bush league owner and front office that was lucky to have him. Do you think he wouldn't trust Stan when he watched him patiently allow Fish five years?
The LA market/media could change how patient Stan is with any potential future HC, but either way the even if he trusts stan to give him time, theres still a risk of poor job security, where as at Michigan thats not an issue. Personally I think theres 2 reasons a college coach becomes an NFL coach. 1. More money, and 2. to prove yourself at the highest level
as for reason 1, thats not really an issue considering Michigan would pay him whatever they need to in order to keep him if leaving every became a real possibility. As for reason 2, IMO he already proved himself to be a great coach at the NFL level, but maybe not getting that super bowl ring does bother him and he does want another crack at it.
I just find it hard to see the benefits of going back to the NFL over his current situation. I feel like theres a reason most very successful college HCs at a premier college choose not to leave for the NFL. The only recent one that comes to mind is Carrol but the pending sanctions likely played a big part in that.