It would be really hard to have a playoff generate as much money as the current system for the schools. The NCAA owns the legal rights to any playoff system and once their monopoly gets its hands on it money would most likely be funneled to side projects.
If there were a playoffs of 8 teams, all of the other bowls would lose a lot of meaning. Instead of, "hey we made it to the citrus bowl, which is still a decent bowl" it is "hey we missed the playoffs the season wasn't good." A lot of fans would be less likely to show up IMO, lowering the money those bowls could generate with ticket sales and tv contracts. Also the bowls generate a lot of charity money as well.
Over the past 10 years the BCS claims to have donated more than $100,000,000.00 to non-BCS conferences.
Also in your plan teams have to travel multiple times. How many times will Penn State fans travel if they made it to the title game? Could they sell out half of a stadium each time on the road?