I think it's more likely that most tracking companies ignore do not track.
And well, DNT could have had legal bearing, since most legislations in the world require you to stop tracking when the user tells you not to.
So, if the user goes and sets up this general purpose opt-out, you'd have to have some sort of argument why you're different than what the user had in mind when they turned DNT on.
Could have had that legal bearing. Microsoft as well as Google and Facebook killed it off pretty well.
Microsoft by turning it on by default in Internet Explorer. Meaning that there were now lots of instances where the user had not explicitely gone into the settings to turn it on (nor did they perform some other action that serves as reasonable sign that this is what they'd want, like going into InPrivate Browsing, or specifically installing a privacy-focused browser / operating system.)
Google and Facebook killed it off by saying right away that they would not respect it. With how many webpages bundle a Facebook Like button or Google: Analytics, ads, GStatic, ajax.googleapis.com, JQuery, fonts, ReCaptcha, Maps, YouTube etc.
As such, there were very few webpages left that could have chosen to respect it and no judge would have just ruled that everyone has to respect it. It would have killed the internet for a few months.
Hell, let's repurpose Do Not Track for it; it's not like it's being used for anything meaningful otherwise.