> I'm pretty sure what spooked them was all the people that were going to be on Edge because it comes with windows,

It was barely 5%-10% of web traffic at the time, it was a minority browser. Statistically no one was using the "default Windows browser" for anything other than "downloading Chrome" for more than a decade by the point that happened. Overall, Windows is itself at something of a perpetual "full market saturation" any given year. There generally aren't massive waves of new Windows users "to be afraid of" and when there are the pipeline of "Google tells people they need Chrome to use the web" convincing new users that they have to have Chrome still seems to be going very strong.

There was no "potential users" to be afraid of. That said, 5-10% of web traffic was enough to get reported as impacting the bottom line in quarterly earnings to shareholders and Occam's Razor clearly suggests that they were worried about existing users of Edge. I do think that would have been a shutdown triggering amount of web traffic no matter how long it took to get to that point and whether or not it was through a browser making it a default or any other means of growth "organic or not".

> Not the people switching to Edge specifically because they wanted DNT.

It was the only notable reason for Edge usage to go up even a fraction of a point in that time period. There weren't major new features. There weren't major new Windows versions or massive PC sales. DNT maybe only influenced "a few dozen" people, but there was a small spike and it did seem to get noticed.

> plus the extremely high chance of bigger browsers doing the same thing.

What "bigger browsers"? At the time Firefox and Safari both had equally small shares of web traffic as Edge. The only browser that was "bigger" in this time was Chrome, and I'm sure they felt pretty safe that Chrome wouldn't do it. They might be afraid that Chrome not doing it might have actually impacted Chrome's huge userbase to consider other browsers for the first time in a decade. If Edge did have a noticeable spike in that time, it would have been for that reason most likely, and that may have been scary to the hegemony.

> I'm not convinced that a feature like that is anywhere near as effective.

I really have had so many websites tell me that "Firefox is an adblocker" despite it blocking zero ads only trackers that I feel it is very effective in practice. Some of these websites (or their ad networks) had marketing teams build entire cute little multi-step animations to show you how to either 1) download Chrome like a good sheep, or 2) click through Firefox's "are you sure you want to white list this awful tracker?" privacy warnings. Anecdotally, from usage experience it's clearly far more effective than either DNT in the brief period when it was working or GDPR have been so far.

(Which the GDPR adds enforceable penalties, sure, but it doesn't enforce it in the user's own browser and the enforcement has the usual delays that a violation would need to be caught and sent to a court to be enforced. We absolutely need more laws in the US like the GDPR, but that still isn't the best solution because it only fixes things after the fact. We also still need strong "before the violation occurs" tools in the browser to find them/enforce them on the user's behalf in the first place because tools like GDPR need time and government intervention to be enforced.)