Overzealous moderator issue was probably the main reason but I think the direct answer and summary from Google directly also had a significant impact on StackOverflow. It took away potential contributors and reduced the incentives for active contribution.

In a way it was a trial and glimpse of what was coming with the AI revolution

I agree it was a moderation issue, but for me it's Reddit that largely replaced my SO usage starting some years ago. Reddit is pretty similar to SO in design, but the more decentralized nature of the moderation means that questions rarely get "closed as duplicate" and answers tend to be more up-to-date as a result. There's not always a consensus answer and I'm often looking across multiple threads on the same thing, but that's still better than an outdated SO post.

> It took away potential contributors

There were multiple times I wanted to contribute to SO but couldn't because I didn't have sufficient "reputation", or something. I shrugged and moved on.