And browsers have had such options (block all cookies, prompt to accept, etc.) built in to them since the very beginning of cookies, except everyone turned them off because they were annoying! So now websites had to add them! And they are just as annoying! ARGH!
A more useful directive would have been some sort of legally-backed "coloured cookie" type system, where a cookie has to declare for what purpose (session, internal analytics, adverts, site-to-site tracking, etc.) it's for, so browsers can then selectively block those categories. That would be useful, because then you could punish people who lie about the purpose of their cookie.