I won't debate the exact moment Group C died b/c the various theories are just a distraction at this point. We both agree that value for money ultimately killed Group C (the fan exposure was there but probably not the Concorde), and value for money is causing the decline of MotoGP.
My main point is that Group C and MotoGP have followed a similar trajectory. If fuel is restricted, braking is wasteful. In Group C they spent bundles on aerodynamics. In MotoGP the tire suppliers spent bundles on tires. In both situations, the exponential cost growth distracted from the fuel-efficiency of engines. In Group C, it turned out that big turbocharged engines were best (not terribly useful for production). In MotoGP it turns out that pneumatics (not useful for production bikes) are required to achieve fuel efficiency and performance. Both series cut fuel capacity rather aggressively, and when reform was critical, both series believed engine capacity changes were the silver bullet.
I don't know what will ultimately happen as a result of 81mm 1000cc 4-cylinder engines, but we have a situation where everyone is unhappy. Neither Honda nor Yamaha want to run 1000cc engines. Suzuki have not committed to run MotoGP from 2012. Ducati's entire program relies on tobacco money. The sport is perched precariously atop the motorcycle racing pyramid, and the poor condition of GP is surely part of the reason Ezpeleta introduced Supersuperbikes (CRTs) as a fall back.
I don't understand what Ezpeleta is doing. I can think of at least a half dozen manufacturers who would be happy to divide up Honda's and Yamaha's marketshare. Honda and Yam aren't even necessary for selling the show b/c brands like MV Agusta, Norton, and BMW can carry the show.