alt Hacker NewsThere was, obviously, only one main reason: LLMs. Anything else makes no sense. Even if the moderation was "horrible" (which sounds to me like a horrible exaggeration), there was nothing which came close to being as good as SO. There was no replacement. People will use the best available platform, even if you insist in describing it as "horrible". It's was not horrible compared to the alternatives, web forums like Reddit and HN, which are poorly optimized for answering questions.
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You overvalue the impact of LLMs in regards to SO. They did have an impact, but it's the moderation that ultimately bent and broke the camel's back. An LLM may give seemingly good answers, but it always lacks in nuance and, most importantly, in being vetted by another person. It's the quality assurance that matters, and anyone with even a bit of technical skill quickly brushes up against that illusion of knowledge an LLM gives and will either try to figure it out on their own or seek out other sources to solve it if it matters. Reddit, for all its many problems, was often still easier to ask on and easier to get answers on without needing an intellectual charade and without some genius not reading the post, closing it and linking to a similar sounding title despite the content being very different. Which is the crux of the issue; you can't ask questions on SO. Or rather, you can't ask questions. No, no, that's not enough. You'll have to engage with the community, answer many other questions first, ensure that your account has enough "clout" to overturn stupid closures of questions, and when you have wasted enough time doing that, then you can finally ask your own question. Or you can just go somewhere else that isn't an intellectual charade and circle jerking and figure it out without wasting tons of time chasing clout and hoping a moderator won't just close the question as duplicate. SO was never the best platform, exactly because of its horrendous moderation. It was good, yes. It had the quality assurance, to a degree, yes. But when just asking a question becomes such a monumental task, people will go elsewhere, to better platforms. Which includes other forums, and, LLMs. So no, what you're attributing to LLMs is merely a symptom of the deeper issue.
It was bad enough that many people resorted to asking their questions in Discord instead which is a massive boomerang back to trying to get help in IRC and just praying that someone is online and willing to help you on the spot. Having to possibly ask your question multiple times before you get some spotty help in a real time chat where it's next to impossible to find again seems unimaginably worse than using an online forum but the fact of it remains and tells us there was something driving people away from sites like SO.
Look at the data - it had already been on the downslide for years before LLMs became a meaningful alternative. AI was the killing blow, but there was undoubtedly other factors.