The way you'd get relegation/promotion to be embraced by enough schools is to give the blue bloods more money than they're getting now.
If Texas and Mississippi State were being paid on the basis of the value they bring to the SEC contract, as opposed to being paid the equally, Texas would be making a shit load more money.
So what you end up with is the death of conferences as separate bargaining entities, and schools pay outs are based on their ratings. The blue bloods in the SEC and Big 10 would have something to gain in terms of more money, the Big 12 and ACC shools would no longer be at a disadvantage to the SEC/Big 10 freeloaders/grandfathered in class, and those schools are the only ones who would oppose it. And I don't think there would be enough of them to overcome the combined energy of the blue bloods and the Big 12/ACC schools, as well as those G5 schools that have a realistic chance at playing their way in.
The only ones who stand to lose anything in this scenario are the ones being most unjustly enriched by the current situation, and I don't think there are enough of those schools to stop it if push came to shove.