It's not like groups of schools have never banded together, left a conference, and formed a new one before.
If enough conference members are leaving in coordination, they can dictate the terms of their departure.
Historically, when that's happened, it's been a group of schools that have a long history and trust built up.
That might exist with some schools in the ACC; maybe. But they'll have to shank some schools with which they also have history and trust. And the big names they need in the group, are (relatively speaking) Johnny come latelies, so they don't necessarily have the long trusting relationship.
The truck stop 12 has some schools with that long history, but not any big enough groups to do it. And, as with the ACC, some of the members of those groups need to be left behind, while some new to the party ones need to be in.
From a financial sense, the universities that would break away probably should do it. But they're universities, and they don't always do the financially optimal thing (although more and more, they are).
Of course, the cynical part of me looks at that last patenthetical and isn't willing to completely write off the idea.