Christ.
Any average slapdick D-1 coach should be able to go 8-5 at Texas, because he'd have good enough players to do so and a not-overwhelming schedule. Here is the average record for the last 3 coaches:
Mack: 10-3
Strong: 5-7
Herman: 8-5
If 8-5 is a "replacement level" coach, then Mack was +2, Strong was -2.5, Herman was 0. That quick and meaningless math actually sounds about right to me. Mack was really good, Strong was really bad, Herman was really average.
We can argue all we want about how they got there. Mack has always been (fairly) criticized for being too conservative, for the OU losses, he wasn't a great Xs and Os guy, etc...but the bottom line is that he averaged 10-3 over 16 years. If you want to say it was due to his talent, that's fine - assembling that talent was his job. If you want to say he was a CEO coach, whatever, great - he was a damn good assembler of talent, and his teams generally weren't stupid and they were generally fundamentally sound. That goes a long way when you have far more talent than 90% of your opponents. We can get into "late-stage Mack" all we want (which interestingly, was 1 Charlie Strong season and 3 Tom Herman seasons), but I'll evaluate him on the entirety of his tenure.
We can argue all day long about what Charlie inherited. It wasn't great. But he also did nothing in 3 years but get worse. Plenty of coaches have taken over worse situations and have turned it around by year 2 or 3. Plenty of coaches adjust to the roster they have, or recruit badasses, or hire innovative coaches...Charlie did none of that. He had a shit AD, yes. But Charlie's teams were consistently bad and showed absolutely no signs of improvement in 3 years.
Herman wasn't horrible; he was just no better than average. We all pretty much agree on his shortcomings, and he too showed no ability to fix them. He's in the meaty part of the bell curve when it comes to most potential hires.
The details are fine for discussion, but this is the bottom line.