Well Mack Brown's last three years were 8 wins, 9 wins and 8 wins. We followed that with 6 wins, 5 wins, 5 wins and 7 wins, so firing a national title winning head coach set us back at least 4 years.
After Charlie's 6 wins, 5 wins, and 5 wins, the next guy won 7 games, so firing a coach that never had a winning season and lost to Kansas in year 3 for someone not as bad (even if only slightly better) apparently moved us a step forward even if only a slight step.
Every coach in Texas football history had a winning record at least one season and at least one winning record in their first three seasons. Every coach since bowls became a popular thing has won a bowl game in their first 3 seasons except DKR and Dana Bible (see there being few bowls when he started). Objectively, Charlie Strong is the worst football coach in Texas history and it is not even close. In his 2nd year he lost 0-24 to a 3-9 Iowa State squad and in year 3 lost to Kansas with the best running back in the country. The only way you would not look at Charlie Strong as the worst coach in Texas history is if you are considering race.