Eternal September was never a problem for SO, it was an asset. Duplicate questions was never a problem for those asking or answering questions (I did both), only for a relatively small group of loudmouthed moderators. Now they hardly have any dupes to worry about but they also have no content to moderate!

The reason that duplicates were treated as dangerous was that SO viewed their most important user not as anyone you have mentioned but instead they prized the lurker most- the person who typed their problem into Google and got brought to SO, and never asked or answered a question because they got what they wanted from that one page load. The entire structure of SO was built around this user.

So why does that mean that duplicates are dangerous? Because of updates. When someone answered a question about how to do something in Python (but it was 2008 so it was written in Python2) SO had ways to get a more correct, up-to-date answer to that question written in 2015 (and then again in 2019) and get that upvoted, and moderators could reward that new answer by editing the original etc.

That is why duplicates were a major threat: if the same question is asked and answered thousands of times, no one is going to go do the work to update all of those answers all across the site. Those lurkers are now dependent on the whims of Google as to which of the many answers you get taken to, and whether it has the latest answer or some answer that stopped working years ago.

And that is why they were so hostile to duplicate questions.

The best was when the duplicated question was ranked higher on Google haha