What happened, what went wrong for it to become like this?
My biggest problem with SO right now is that the most popular questions are outdated and temporally incorrect. A big problem when you are trying to learn a programming language that has evolved considerably in the last 10 years. Go ahead and take 3 common beginner-level tasks in Javascript and Google them. The first result on at least two of them will have been last updated in, say, 2012. Even if the modern answer is there, it won't be the accepted answer.
"Things have changed ... What happened, what went wrong for it to become like this?"
Has this not essentially always been the critique of StackOverflow?
I'm still glad when google sends me to a good stackoverflow answer, because it's usually high quality and helpful. I don't ask many questions there, so it's hard to know if the problems are serious.
The code is neat. The person clearly says what they tried. It is an answerable question. Maybe it would be better or more useful if it were a wiki-able question. But also this person was new, and was not guided in any way how to improve the question.
There's a review queue for cleaning up first questions but all these moderators don't give a damn about that. They're just mods because they want to hurt people.
They thought about the system really well. It's based off HN's. And it worked. HN's system still works.
Years later, they removed the reputation penalty from downvoting questions. They trusted the community to be mature with this new privilege. And it all went to hell.
I don't have the data for this. I found the article above while searching for it. But this is the hypothesis.