Nebulous things like "prestige" aside, let's look at the money.
UT will probably (and definitely COULD) pay their next coach $6m/yr. Saban is making $5.3m/yr now, and I would not be at all surprised if Texas tops that for a coach. They want the best, and can afford it.
So, let's say they pay $6m/yr. That's more than all but the top 10 NFL coaches make in a year.
However, that's $500k LESS than Kelly is making at the Eagles. Frankly, I have a hard time believing that he would leave NFL for less pay in college.