Keeping Ash makes a ton of sense if you're into logic and reason, instead of knee-jerk reactions. Here's my reasoning:
(1) Wisconsin has won 3 outright Big Ten Championships in the last 50 years: 1999, 2011, 2012. Ash was their DC for two of those.
(2) Ash won a National Championship as Ohio State's DC.
(3) Urban Meyer, who is credited as inventing the RPO that is all the rage today, calls Ash "the best defensive coordinator in college football". And mind you, Urban has employed possibly the best combination of DCs in the last 20 years: Chris Ash, Greg Schiano, Luke Fickell, Alex Grinch (current OU DC), Greg Mattison (current tOSU DC), Charlie Strong, Tim Beckman, and Everett Withers. Only Saban can claim as strong of a lineup. Yet out of all of those guys, he calls Ash "the best in college football". He's said it from the first day he was hired at Texas. He didn't have to. He could have called him "one of the best". But, he didn't mince words.
(4) Texas was robbed of approximately 1,500 spring and summer reps, which unquestionably hindered not only the rollout of the new defensive scheme, but resulted in the defense entering the season needing a ton of in-season reps to get comfortable.
(5) The Texas defense started off horrible. But, once they caught up on the reps they were deprived of in preseason, they started to turn in better and better performances, holding 4 of the final 6 opponents below their season scoring average, and allowing 2 just a few points more than their season scoring average.
(6) Even with starting the season lacking comfort in the new defense, and ranking VERY poorly through the first 4 games, they played well enough down the stretch to deliver some pretty good good defensive metrics:
2020 Texas Defensive Rankings (out of 127 teams)
15th -- Points Allowed per Drive Starting at Opp 0-20 Y/L
28th -- 3rd Down Conversion %
29th -- Yards per Play
30th -- Yards per Rush
32nd -- Busted Drive Rate (% of drives that earn 0 or negative yards)
35th -- Yards per Pass Attempt
40th -- Punts per Play (how frequently punts are forced)
40th -- DFEI (defensive football efficiency index)
41st -- Expected Points Added (points added/lost by the D)
41st -- Punts Forced to TDs/FGs Given Up
41st -- Points Allowed per Play
49th -- Points Allowed per Drive
Nothing was omitted for the sake of making the D appear better than it really was.
(7) Continuity matters. Familiarity matters. Comfort in a scheme matters. All of those affect the win/loss column, especially in the early going of a season. That's why Saban was 7-6 his first season at Alabama and 8-4 his first season at LSU, they blew up the 2nd year at both. It takes time to get your players comfortable in what they need to do on the field. If Sark fires Ash, then the defense will go into the off season needing to learn their 3rd scheme under their 3rd DC in 3 years. You think that's going to help the win/loss column in year 1?
(8) The 2021 recruiting class is already a disaster. And, Sark's results at UW and USC unquestionably will be used against him in recruiting. If he fails to deliver a 10+ win season in year 1, it will be a confirmation of his prior HC results in the minds of recruits. Texas can't afford a second down recruiting class, because those are the guys Sark will need to deliver results in years 3 and 4. The only way to sign a great 2022 class is if the win column justifies those recruits spurning Alabama, LSU, Ohio State, OU, A&M, et al.
(9) Covid isn't over. And, we won't have herd immunity until summer at the earliest. That means, it's very likely we either don't get a spring season or the spring season is heavily modified to limit spread. Either way, it makes no sense to roll out a brand new defensive scheme when once again, players will likely be deprived of vital reps, especially when Sark/Texas needs a strong first season.
(10) The known options for replacing Ash are either a DC who's coordinated a whopping 13 games vs P5 teams in his life or a DC that has struggled shutting down teams with Alabama-level talent + input from Saban.