It's important to note that Cookie banners and GDPR are separate things and that these pop-ups are covering both purposes in a lot of cases. So even with a DNT flag, many sites will still need explicit, advance permission to process data, and cookies will still be a method for technical enablement.

In an ideal world, respecting DNT would instantly bin 95% of the cookies and data processing requests, but I'm still getting automatic (and permissionless) marketing subscriptions from companies when I make purchases, and the British ICO seems unwilling to intervene, so it seems unlikely that DNT being case law is going to have any quick effect on things.