I think a pro for playing more P3 vs P3 games would be a far bigger potential audience.
Almost no one in the B1G or P12 states cares about a typical ACC conference game, say Syracuse vs Wake Forest.
Make it Syracuse vs Michigan or Syracuse vs UCLA and I think it is highly likely the ratings for the typical inter conference games will be higher than typical normal conference games. Probably significantly higher.
That would get ESPN's interest.
My guess is that if the alliance members went to ESPN and asked what could be done to get more money, ESPN told them the more inter conference games played, the more money paid. What if the ACC schedules went from 8 to 6 conference games, with 3 games each against the other 2 conferences?
I would think that might lead to a lot of additional revenue. It would be devastating for the MAC, the AAC, the MWC, and whatever the B12 refugees call themselves. But it could get the Big 3 a lot more money.
Could it be enough for the ACC schools to stop playing their SEC rivalry games? Hard to imagine them stopping. But it was hard to imagine Pitt not playing WVU every season. Or Texas not playing Texas A&M. Or Oklahoma not playing Nebraska.
If the money is right, most anything is possible.