Yeah, sure, your average end-user is totally going to do that.
That's the difference between the two scenarios. A sysadmin will know what to look for and will know how to appropriately react to it. Your average end user probably doesn't know, care, or know how to react to it. And a built-in browser implementation will never happen because all the major companies have it in their best interests not to implement such a feature. If that weren't the case, we'd have had that feature long ago.
I doubt an average sysadmin would ever notice, let alone knowing what to do about it, let alone putting in the time and effort to do it.
Anyway I'm not sure I put the responsibility on the right group. It will probably be down to websites choosing ad networks that respect their users' DNT settings. Just like they choose ad networks that don't host malicious ads or ads that take over the whole page.
adBlock has millions of users. If DNT decreases the adoption or even slows the adoption of adBlocks that's already a big win for adNetworks. You don't necessarily have to track a user to display ads, it's just possible to do more effective advertising if you do so.
That's the difference between the two scenarios. A sysadmin will know what to look for and will know how to appropriately react to it. Your average end user probably doesn't know, care, or know how to react to it. And a built-in browser implementation will never happen because all the major companies have it in their best interests not to implement such a feature. If that weren't the case, we'd have had that feature long ago.