Frontier probably underwent its largest changes during his time. Transition from Republic ownership to Indigo. Transition from flat finances to record profits. Transition from a LCC to ULCC. Transition from decent airline to an airline with one of the worst rate of passenger complaints and on time performance.
Whoever replaces him needs to clean up the operation and fix the issues with poor station performance. The contract staff we use in most cities needs to be addressed....big time.