Hmm... Let's see. Nintendo looks to make games that sell 10m-20m on Wii by appealing to everyone. But instead this Sony hack would have them divert resources to make a Mature game that would be lucky to sell 3m to create a market for Sony?
Sure that'd be nice for Sony but what's in it for Nintendo? I'm sure the big N would rather use those resources and make another 20m seller instead. It'd be more profit than the cut from all of the 3rd parties M efforts that would follow's Nintendo's M game.
Nintendo is not in the habit of splitting up it's audience and appealing to one chunk at a time. They make games for all. Why 3rd parties insist on doing it the other way, and then asking Nintendo to do the same is beyond me.