I don't think it was ever meant to be a comprehensive solution; it was meant as a start to address the rampant issue of tracking. There wasn't a mechanism short of outright blocking for a user to declare intent. The argument has always spiraled around what is the agreement between a user and a site they visit. Implicit agreements, revenue models, etc, but it's always been a unidirectional communication; the website tells the user, after the fact, what the terms and conditions are.

Do Not Track was an attempt to give users a voice and the overwhelming response to it by the trackers was to ignore it. DoNotTrack wasn't an effective anti-tracking solution, but for every opt-in, it was a clear message from users. It's why Microsoft's auto-opt in, while probably well meaning, really screwed the whole thing. It had to be a choice.

It was a message, and it's still a very strong argument when it comes to the discussion about dishonesty among trackers and advertising networks.