Firing a coach that has never had a major scandal or even had a losing season looks insane to prospective candidates (which usually means you are going to need to hire someone who has no better options, i.e., an unproven candidate). With some of the schedules coming up, having 7, 8, and 9 win seasons is definitely possible (even losing seasons are possible). Smart candidates (and don't we want a smart coach?) will realize that NEEDING a win percentage better than Bronco's, while certainly not impossible, is certainly a difficult proposition.
The only way it works is if you have a new AD. Then the new AD can at least say, "Bronco was not my guy, I want my guy and that guy is you." Holmoe cannot say that to prospective candidates.