If the P5 breakaway and are only playing themselves, first, the tourney games will duplicate a lot of regular season games. It will look a lot like a Big Ten - ACC challenge, only at the end of the season. Second, it will be a smaller shorter tourney. With only 65 schools, half with losing records, it will be hard to select more than 16 or maybe 24 schools. That's a 2 week tourney and interest will be limited to 16-24 fanbases. Moreover the same teams will be in every year, so it will get repetitive.
Meanwhile there will be a separate tourney for the non-P5, and so we'l have two champions. This will create demand for a super-championship between the divisional champions.
What would create more interest is if at some point you merge the tourneys -- the P5 Final Four and the non-P5 Final Four get matched for an 8-team tourney. But then the P5 would have to share the money.
I don't know that it would generate much more money for the P5. Maybe, but the NCAA tourney credits are already distributed largely to the P5. And since the P5 have the big network contracts, they will have a big lead in exposure and that will give them a bigger advantage in recruiting. I think they might find they're getting all the money there is to be had with the current basketball structure.