Part of the issue is that it's not just a position coach that develops the player.
The S&C coach (as well as the training staff and the dietician,(s)) is a huge component to long term development and Sark hired a third tier S&C guy. There's not a modern and multi-faceted approach to full body flexibility and explosive power nor any cutting edge analytics that tailor not only the S&C workouts but more importantly full body recovery that is specific to each player and position group, as well as optimum recovery from nagging minor injuries. The gap between where other schools (specifically Bama, Clemson, tOSU, Oregon) are with S&C and Texas is with Torre Becton is a vast chasm. It's a FAR bigger gap than the gap Sark vs. other QB developers on those staffs would have as an advantage. Similarly, Sarkisian hasn't added former HC/OC/DCs to any of the support roles to speak of, much less an army of them. At Texas there aren't former HC/OC/DCs grading each days position group film, looking for things to work on in the next practice/tells that give away assignments, or grading the effort and drills of those position coaches and their army of assistants coaching along (regardless of the rules). Nor are you going against an entire defense of NFL talent for every rep. For 5 years. Even when it's 3s vs 3s.
If you ask all of the players, the trainers, the high school and 7v7 coaches if they think the recruit will be developed at Alabama, independent of who their position coach is or will be, they will unequivocally say "yes", because that has proven to be true. No matter the position coach. No matter the position. The system is in place. If you ask even just QBs if they will be developed at Texas under Sarkisian you won't have unanimity and you'll have more than a few tell you that they aren't sure he'll even be the HC by the time this 2022 class finishes. You'll get the same Alabama-type responses at Clemson and tOSU. Beyond those schools it drops to a more position-by-position basis for other schools (like OU, UGa, etc) and you see the clearly defined gap between the top 3 and everyone else.
The reality is that Sarkisian is a mediocre head coach taking over a program mired in mediocrity. He's done nothing but make mediocre choices for his recruiting and support staff. All while ALL the other schools have preached since Sark's hire that this is Texas doing "more of the same" -- being mediocre. You can't counter the "more of the same" messaging from EVERY OTHER SCHOOL with "We did X at Bama and developed Player Y" because all of these recruits/trainers/parents/coaches have seen an army of coaches go from Bama say excatly that and do exactly fuckall at their new jobs. So you have do it by showing them that you have changed not only what you've done in the past that was decidedly mediocre (nope) but how the entire org works from the top down to be on par with those top 3 schools. (nope).
It's not as easy as saying I coached Mac Jones or Alex Leatherwood - because ___________ (insert 30 coaches who have come and gone) coached ______________ (insert 70 names from Alabama since 2011 when Saban's recruiting classes started hitting the draft like a fucking hammer). Sarkisian and his staff will have to prove that development travels with them because the messaging has been countered by every single school in the country using the "more of the same" messaging against Texas.
Sarksian is going all-in on scheme and preparation. If he succeeds he will be able to quickly sell this staff's ability to scheme, develop, and identify talent and it will resonate. He will be able to flip some guys and really hammer the 2023 class. If he loses 3 or more games that all-in approach will net him a pink slip before he ever really gets going because the 2022 and 2023 classes will be marginal and we will be doing this dance again in 3-4 years.