I don't know...I think once the market shares reach a certain equilibrium, like your example of 30/50 that most publishers will have to make their games cross-platform to reach the majority of gamers, no longer does it makes any sense to be exclusive.
So then it comes down to choosing the the base platform, I think that ease of development would be the deciding factor in that case, I wouldn't be surprised to see many use 360 as base platform since it's installed base can be counted on to be a very large percentage of the market, and the development tools are reportedly extremely good.