Tom Herman is this year's Justin Fuentes--he's the 'hot coach' that will be on everybody's list when the coaching carousel starts turning in November. I doubt very seriously that he stays in Houston.
Ok, for the sake of argument, let's assume:
1 - It is announced that Houston will be admitted into the Big XII.
2 - It is announced that the Big XII will then resume having a conference championship game.
3 - The Houston AD makes it clear that they will want to keep Herman at the helm and will do whatever they can (read engage in a bidding war) to keep him.
4 - The current and future influx of cash from the Big XII conference would enable Houston to pay Herman close to whatever any other school might offer.
Given those conditions, what potential opening do you believe would represent greener pastures to Herman?
LSU is a big name but would that really be a better situation? Financially, LSU wouldn't be able to go over the moon paying him significantly more than Houston. While LSU is in a hotbed of recruiting, so is Houston - and he would have to compete with both SEC teams and Big XII teams, including the newly joined Houston, trying to raid Louisiana for those recruits. Finally, pretty much regardless of how the Big XII decided to structure their divisions, Houston's division would be much less competitive than the SEC West.
Texas A&M? Pretty much the same as LSU except A&M might be able to pay more.
The Barn? That's most likely a worse situation than the first two.
As for outside the SEC, maybe USC if Helton can't turn around this season? It's certainly a blue-blood name and recruiting shouldn't be that difficult. However, the path to the CFP for USC in the PAC 12 is probably more difficult than for Houston in the Big XII.
Without knowing Herman's personal motivations, I don't know how you can say any potential job opening would be more alluring to him than Houston. Maybe he likes the idea of coaching at a traditional power and/or in a stronger conference. Maybe he doesn't care about that and his dream is being an NFL head coach. I don't know.