The moment you notice GA is present in about 50% of top websites you notice that useless GA cookies going around the internet represent 12% of all HTTP requests.
This is incorrect. "GA is present in about 50% of top websites" does not mean "50% of HTTP requests are on the domain with the cookies." Many websites load external images, etc. See ytimg.com, etc.
Of course the websites that use cookieless domains for resources reduce this issue by an order of magnitude, but it still exists in at least the html document request. Besides if you think that 50% of the traffic of the top million sites probably represent 70%-80% of the total internet traffic the real gain may be even larger.
1) How can half the traffic on a subset ever represent more than half of the traffic on the superset? The 12% number assumes that these top million sites have effectively all global traffic; in practice it will only ever be less than that.
2) These top million sites are exactly the sort that are likely to serve static content from a different hostname.
This is incorrect. "GA is present in about 50% of top websites" does not mean "50% of HTTP requests are on the domain with the cookies." Many websites load external images, etc. See ytimg.com, etc.