It's so dependent on individual motivations. Just because Bronco has assistants that enjoy what they do, make a great living, have relative security in the college football business, and are successful doing what they do, doesn't mean they wouldn't be good head coaches if they decided to do that at some point.
Also, pretty tough to compare 2 guys who've coached half the number of years LaVell did. Plus, you included guys in LaVell's 'tree' that never coached for him but only played for him. You're comparing apples to oranges.