I think you're severely underestimating both the revenue that the P5 can achieve on their own and how much they're giving up financially to the entire rest of the NCAA by participating in the NCAA Tournament. There are probably 15 to 20 auto-bid shares per year that are granted to conferences that are far beyond the market value of such conferences. Also, remember that the NCAA Tournament is subsidizing ALL levels of the NCAA - Division I through Division III. Those expenses for every single sport for every single NCAA school at every single level vastly outweigh any consternation that the P5 would have about running their own tournaments for non-revenue sports... which is what they want to do via conference realignment anyway!
Sure, there are excellent G5 basketball schools, but don't let that blind you to the fact that the P5's experience with basketball revenue sharing is EXACTLY why they want nothing to do with the same type of system in football. They'd be more than happy to marginalize any non-P5 school for basketball just as they've done in football and, in turn, increase the value of the P5 basketball regular season that they can keep 100% to themselves (which is what they've also done in football).
I'm not saying that the P5 is going to actively disband the NCAA Tournament. It's still a huge bridge to cross even for the biggest money-grubbers in the sport. However, if it comes down to a choice between protecting football autonomy or the NCAA Tournament, the P5 will choose football autonomy every single time without debate (and take basketball along with it in the process).
I don't think you are correct. If the P5 broke away in basketball, say the G5 (or others) don't play them in basketball anymore. It would mean that half (plus or minus) of the P5 teams would be under 0.500. I guess you could have a basketball tournament with all P5 schools, but you would have some really ugly games.
And, basketball is not football. Having a P5 only tournament would exclude many of the top basketball markets in the US and be skewed heavily towards football markets. For example, only one New England team (BC, which nobody cares about in New England) would be eligible. And only 1 school in NY and New Jersey. No Philadelphia or DC schools. But, we would have 2 schools each from Mississippi, Alabama, South Carolina, and Tennessee. Although a P5 football league may not attract political challenges, I would GUARANTEE that an exclusionary basketball tournament would attract tremendous political pressure. Just think about this: Harvard - out. Penn - out. Princeton - out. Georgetown - out. Villanova - out. Temple - out. Memphis - out. Providence College - out. UConn - out. UMass - out. All SUNYs - out. Virtually all religion affiliated schools - out. Think about this: the alma maters' of the President, VP, Attorney General, Senate Majority Leader, and the Speaker of the House are out!
In the past 11 years, the non-P5 schools have placed 12 schools in the Final 4, has had 3 champions and 3 runners up.
I guess you could have the NCAA Tournament and the NIT like we had in the past, but I would think it would dilute the product for everyone. Yes, you are correct in that the NCAA derives 90% of their revenues, or $500 million from the basketball tournament. But, it basically means that college basketball, not football is subsidizing all sports in the NCAA.