I don't understand how it was not fruitful.
In 2015 UNT was dead last in the entire FBS in offense and 5th to last in defense led by a defensive minded head coach.
Since then, Littrell and his offense gave UNT the best chances to win championships and football games, regardless of bowl game status. Two of the best seasons, record-wise, in history. It took North Texas to 6 bowl games from 2016 to 2022. Prior to that, North Texas had been to 6 bowl games since 1959. To me, that is fruitful.
UNT had a guy that ran a balanced, successful offense that transformed when Mason Fine left to add TE wrinkles. Hired an experienced defensive coordinator with former head coaching experience. Everything fans are currently asking for, I feel like. The results were not there and instead of letting him work through it, powers that be got rid of it.
The grass is always greener... but football and offenses have left the wishbone, the swinging gate, the triple option behind.
If that is the criteria for the next head coach, you may immediately eliminate some of the best options available.