Texas's issue is the Defense, their Offense can move the ball pretty well & have no problem putting up points, they just can't stop anybody.
I don't think Herman is necessarily a bad coach, he's 49-20 for his career & 4-0 in Bowl games, he's just not good enough to get Texas over the hump & be playoff contenders, because of the lofty expectations any time they lose it gets amplified because UT believes they're supposed to be on the same level as Bama, Clemson, Oh St, LSU etc..
Herman has had 4 winning seasons in 5 & quarter years as a HC (this year will be 6), he's not terrible like Muschamp or something, he's just not a championship coach.
He's a good recruiter, but he continues to miss on the very best players in TX, he gets his fair share of really good ones, but the elites are mainly going to Oklahoma, Bama, LSU, Clemson & other programs.
Unless UT decides to go with another hot shot G5 coach or go with a highly touted coordinator with no HC experience I don't think they fire him because I doubt they can steal a top HC from another program. If you're a HC that already winning at your program, unless UT offers to quadruple your salary what incentive would you have to go their knowing they'll fire you if you don't win a Natty in 3 years?
I think Texas needs to decide if they're going to try to actually have a Defense in the Big XII where it's very difficult to actually have a good Defense because of the style of teams you have to face every week, or just embrace the Air Raid fully & disregard Defense completely & just focus on out scoring opponents every week.