Bronco would be considered a great coach... because what he has accomplished today would have been relevant back then. Problem is... what BYU did accomplish then still outshines what Bronco has accomplished today, it's about the ONLY thing that keeps BYU even slightly relevant today.
30 years ago... Utah's 2008 and maybe there 2004 season would have gained them a National Championship. Boise State, with there success if it were 30 years ago... would have seen a National Championship. Bronco hasn't came remotely close to what Utah accomplished in those seasons... and they did it as a MWC team.
It doesn't matter that the 10 year stretch is the most successful since 1980... it still doesn't meet up with the bar that was set, and coming close to it is hardly an accomplishment, especially when you consider what is relevant today.