Sure, that might change if Utah was consistently meh or worse, but right now, they would absolutely fill up a bigger stadium. There is PLENTY of demand for Utah football tickets. Any suggestion otherwise is just head-in-the-sand nonsense.
Also, you are wrong about the reason for their basketball arena not filling up. People don't show up there because the team is not good, and it hasn't been good for years. There is reason to believe this would also happen to the football games if Utah started sucking.
Fans not showing up to basketball games has nothing to do with "good" fans not wanting to be around "bad" behavior.