What you're suggesting is basically the same thing as what the law is achieving. Yes, users could block all cookies but this will break functionality on a lot

What you're suggesting is basically the same thing as what the law is achieving. Yes, users could block all cookies but this will break functionality on a lot of sites, so it's not reasonable. And yes, sites could communicate which are functionality cookies and which are tracking cookies, but as you say it's hard to police this, so pushing the issues to the user's software won't work. What the law does is fixes all this by requiring sites to obtain consent in certain scenarios; but if your site only sets cookies required for the site to function (shopping cart, login cookies), or if it tracks users for the purpose of security (eg a bank that detects when you log in from a new device / location) you DO NOT need to obtain consent, no banner required.