What I always appreciate about SO is the dialogue between commenters. LLMs give one answer, or bullet points around a theme, or just dump a load of code in your IDE. SO gives a debate, in which the finer points of an issue are thrashed out, with the best answers (by and large) floating to the top.
SO, at its best, is numerous highly-experienced and intelligent humans trying to demonstrate how clever they are. A bit like HN, you learn from watching the back and forth. I don't think this is something that LLMs can ever replicate. They don't have the egos and they certainly don't have the experience.
Whatever people's gripes about the site, I learned a hell of a lot from it. I still find solutions there, and think a world without it would be worse.
SO also isn't afraid to tell you that your question is stupid and you should do it a better way.
Some people take that as a personal attack, but it can be more helpful than a detailed response to the wrong question.
Banning advertising targeted at the user rather than the context
Enforcing Do Not Track
Enforcing GDPR (especially sites that use cookie banners)