A blacklist seems like a pretty effective strategy and allows them to deal with offenders simply. I'd imagine you're not advocating for them to add proprietary extensions to P3P. What would you suggest they do otherwise, out of curiosity?
I think MS probably needs to acknowledge that P3P is broken, and change the default so it doesn't affect third party cookie acceptance. Administrators for Windows environments that think otherwise can override the default by deploying a group policy.