The only reason ad networks were behind DNT in the first place was because they thought no one would use it and it would be a cheap way to divert attention away from legislation or technical solutions like adblockers. The moment there was any significant uptick in usage for any reason, there would have been a competitive motivation for the networks to abandon DNT.
On-by-default wasn't a problem because it was some kind of fundamental paradigm shift, it was a problem because it meant that the setting would actually be turned on for normal people.
If it were actually about Microsoft breaking a sacred contract or something, it would not have been difficult for ad networks to detect the IE10 agent string and apply browser-specific policies. But for the most part very few companies ever did, because if you're an ad network and you realize you can get away with ignoring privacy settings on one browser, why on earth wouldn't you do the same for every other browser?
DNT was doomed from the start specifically because it relied on advertisers voluntarily participating, and advertisers are never going to disrupt their own business model voluntarily.