Perhaps the opposite approach could cut down on meaningless clicks

I don't think the defaulting of tracking cookies to "off" does have anything to do with the DNT header. Websites doing so simply follow the law as a decision by the European Court has made clear in ECLI:EU:C:2019:801:

"On the basis of those explanations, it should be noted that, in accordance with Article 5(3) of Directive 2002/58, Member States are to ensure that the storing of information, or the gaining of access to information already stored, in the terminal equipment of a user is only allowed on condition that the user concerned has given his or her consent, having been provided with clear and comprehensive information, in accordance with Directive 95/46, inter alia, about the purposes of the processing.

[...]

Thus, as the Advocate General stated in point 60 of his Opinion, the requirement of an ‘indication’ of the data subject’s wishes clearly points to active, rather than passive, behaviour. However, consent given in the form of a preselected tick in a checkbox does not imply active behaviour on the part of a website user."

http://curia.europa.eu/juris/document/document.jsf?docid=...

Thus it can be said, that there is now no longer any doubt that tracking EU users is only legal if they explicitly gave their consent by clear and unambiguous action, e.g. opt-in. The only exception is if tracking is necessary to actually provide functionality to the user, e.g. login sessions.

The EU is really a gift to Europeans.