I work in web analytics. Recently, one of our clients decided that they wanted to track visitors across multiple domains. After much hemming and hawing, and res

I work in web analytics. Recently, one of our clients decided that they wanted to track visitors across multiple domains. After much hemming and hawing, and research proposals, and industry comparisons, the ultimate solution that was decided upon was... third-party cookies. The change is extremely simple, and the data loss from people who block third-party cookies is like 5%. So what prevents sites from deploying these technologies? Economics. To your point, there are no insolvable technical barriers. But right now the marginal return return on these technologies in no way stacks up to the marginal effort required to deploy them. Considering the fraction of the population that even knows what a cookie even is, I do not anticipate this changing in the near future. If everyone starts blocking third-party cookies, then yes we'll see an uptick in use, but I don't see that happening this decade.