That compromise would require knowing who someone actually is between site and site - you could collect the full list of cohorts but you'd need additional infor

That compromise would require knowing who someone actually is between site and site - you could collect the full list of cohorts but you'd need additional information to be able to marry all that data together - let's say something like facebook injecting JS into every page on the internet to harvest cohort information and then gleefully reselling that information.