To be 100% honest, the OHL is a developmental league for coaches just as it is for players. I am of the opinion that legacy OHL coaches that compile significant win% over long periods of time is overrated to some degree. If the coach was very good and the measuring stick is longevity, win% and Championships, I am not sure looking at many of the coaches mentioned is as clear as it may seem.
If there is a good coach, that coach graduates like players. For example, take a look at DJ Smith. NO ONE is going to mention DJ Smith in the same breath as Brian Kilrea, Bert Templeton or Dick Todd. But, DJ was a successful coach for two franchises (Windsor and Oshawa). he may have only been an assistant in Windsor but they won two OHL Championships and the Memorial Cup both years and in three season with the Generals, he won a League Championship and Memorial Cup. Then he graduated to the Toronto Maple Leafs bench as an Assistant and then to the Senators as a Head Coach.
The DJ Smith trajectory “should” be the trajectory for all coaches. So, although Marty Williamson is a successful OHL Coach with 9 years behind a Barrie bench and 6 years behind the Niagara bench, if he were such a great coach, why no opportunities at the NHL level? That is not meant as disrespect either. Just a honest assessment of how we seem to revere long time OHL coaches when those coaches should be graduating to bigger and better leagues and not coming back.