Honestly, as an MLS fan, I'm probably just taking my frustrations out here because I'm sick of all the people that think pro/rel is a cure-all for everything.
The Premier League is what it is for a few reasons -- first, it's far and away the best league in the anglosphere, and the FA is the longest-lived association in the English-speaking world by far. The FA, formally speaking, invented the sport. There's a lot of momentum there. The spanish-speaking world, on the other hand, has long-lived leagues in Mexico and Argentina. Brazil's league is 103 years old. On the other hand, the second-oldest major English-speaking soccer league probably MLS, founded 1994. For upwards of a century, if you were a soccer fan that only spoke English, you followed the FA's top flight, and that encompassed two of the largest economies in the world. That made them rich, which made them powerful. It had little or nothing to do with competitiveness.
Calling the Premier League "competitive" outside the top 6 or so teams is laughable.
College football is, in large part, about their traditions. Michigan and Ohio State are far more valuable together than they are separately, largely because The Game is going to draw the largest ratings of any regular-season game either of them will play. Rivalry games make this sport. Minnesota might not be valuable on their own, but Minnesota-Iowa and Minnesota-Wisconsin are both worth something. The Big Ten is betting that Rutgers and Maryland will both develop rivalries that grow to that level. I think they're wrong, particularly about Rutgers, but that's neither here nor there.
The obvious answer is just to schedule them as nonconference games when one of the teams gets relegated, but with schedules being set years ahead of time, that's not really feasible.
Even ignoring the fact that income is distributed by conferences and we'd run into the problem of figuring out what portion of a conference's money, this still runs into a lot of problems.
First, I think there's a good argument that Baseball is more broken than Football because of the whole financial aid disparity present in that.
Second, pro/rel doesn't fix this problem, it worsens it. If you're a recruit, how do you commit to a school when you don't even know what conference you're going to be playing in? In a pro/rel system, how does a coach get a top recruit when there are questions about whether the team will even be able to stay in the league? If I'm Tom Herman or Lincoln Riley and I want a recruit that's committed to Tech or Baylor, all I have to do is raise probably-valid questions about whether they can maintain their spot. That means talent gets more concentrated in the top few schools.
An expanded playoff is a way better solution if you want to fix the broken system.
Not really. Even a top-flight Notre Dame still probably loses games against a handful of their rivals.
So...how do you split the SEC/ACC divide? Whichever conference loses their schools in Florida and Georgia (because those two states are probably a package deal, since you're not splitting up the UF/UGA rivalry) is going to be financially gutted.