Ben Johnson is a hot commodity because of the Lions' success the last 2 regular seasons (playoffs, not so much). He's not a "top tier" HC. He's never been a HC. We do not know how he'll do as a HC.
The 2 biggest OC hot commodities I can think of were Norv Turner and Josh McDaniels, both coming from record setting, 3-time SB winning teams. They both failed at HC in multiple places.
Of course, there are other examples of OC who were hot commodities and had success, as well as not-so-hot OCs who had great success (like Payton) but it just comes to show you, you never know how things are going to turn out.