Lets assume the trade off isn't to improve DR / or noise... that it's all about we want megapixels, and that introduces extra noise, so you're stuck with a lower ISO.

If its a 5D that can't handle bad light and requires bigger CF cards, bigger hard drives, and needs more processing power, to get extra detail that most people don't need, then it's a really tough sell. Would you release this camera? The people who want 'crop reach' just upgraded to the 7D2, so it's not about up-selling that market into the 5D price range. The 'need/want' 50MP market is too small, isn't it? They *have* to have improved something, and I think that its got to be noise and DR.