Once you analyze this from the standpoint that this is naked capitalism with no guardrails, it's the only thing that makes sense. If the B1G and SEC get to a point where they believe they can shut everyone else out and credibly claim the true national championship is from within their conferences, they will because they'll make more money. If the #3 conference has a handful of strong enough programs, then they can't credibly claim that and they probably won't do it. So, for example, if the ACC manages to keep FSU and Clemson, and grow SMU into a top 25 program and get another school or two into the top 25 on a rotating basis, they're pretty hard to shut out while still claiming to have a legit national championship. But if they get raided and lose their 5-7 best schools, it's pretty easy to shut them out.
It won't happen immediately, but if this process continues to play out unchecked and if the ACC and/or Big 12 don't make the right moves and get a little lucky, it'll happen. You just have to stop thinking about it in terms of what's best for the sport, the fans, the players, etc, and think of it from the standpoint of what makes the B1G and SEC the most money. It's obvious IMO that breaking away makes them more money, and it's blatantly obvious their guiding mindset is making the most money.