They don't need to leave the NCAA as the NCAA doesn't really receive any of the football revenue anyways. Without the G5 schools like Ole Miss, Rutgers, Boston College, etc would instantly become annual 1-3 win teams.
The value the G5 has to the P5 is it allows their middle and bottom tier schools to pad their win total enough to keep fans interested. Perennially winning 1-2 games a season kills a fan base. We've witnessed that first hand at Akron. The Big 10, SEC, etc don't want that for their fans. They would rather just keep throwing peanuts at the G5 to keep their fans engaged.