This is part of the better long-term solution for online privacy. Spend the effort to educate users about privacy and tracking and build easy to use software options, rather than investing time in standards that don't get adopted and are based on trust.
The voluntary standards were never going to work since the interests of the parties involved are diametrically opposed. The voluntary systems only work for the ad companies if a very small number of users take up the option and if privacy remains a niche issue. As soon as developers looked to apply DNT broadly, they baulked.
Trust is also an issue. Facebook previously said that although their share buttons are hosted on the same domain as facebook.com, meaning cookies are sent - that they are not storing or tracking user web browsing data. They now are using that data, and very little was made of the reversal.
The same reversal or business changes could turn on DNT in the same way. With DNT the sites still receive the cookies and can still store the data. They are able to comply with DNT for a period and then later reverse their position and still use the old data for targeting (and a DNT header is one more data bit that says a lot about a user).
There are now a number of competing opt out standards. None of them really work and they each have problems, but companies need to be able to say they let users opt out whenever they announce a new tech that encroaches further into our lives (Facebook use aboutads, the NAI has their own, Google has its own, most of the aggregators have their own, there are other efforts to sync opt-out).
The solution is to cut the problem off at the head and solve it with software. Browsers and that are easy to use in terms of specifying who you trust (like installing an app) and that don't ever make third-party requests by default. Now that the pretense of negotiating a solution has been dropped, effort can go into developing better software control for users and tech solutions - since none of the current offerings are perfect (which is excusable).