If those top 32 (actually ~70 for the top 5 conferences) programs broke away, there wouldn’t be enough of them willing to give up a home game. If all 70 of those schools want to host 7 or even 8 home games on a 12 game schedule, you’re going to need ~35 schools or more to give up a home game or two.
Not every team in the major conferences would go, only the top grossing 30 or 32 teams.