Really, when you look at the entire Mumme/Leach coaching tree, the only relatively successful coach at a high level has been Leach, and he's not exactly a world-beater. In 14 seasons, he's tied (3-way, lost tie-breaker to play in the championship game) one time for 1st place in a division, and tied for 2nd in his division twice.
Granted TTU and WSU are not plum jobs, but compare that to what Briles and Patterson have done at "second-tier" Texas schools and it's not hugely impressive.
That's a long lead up to, even if Holgorsen returns, I don't think bringing in someone who brings nothing but immersion in the same general system and concepts is likely to help much. What would probably help a lot more is someone with a different perspective who could be helpful in making needed changes, and making those changes work.