> I think Stack Overflow went dead because of AI specifically

This doesn't hold up when looking at usage charts. There is a clear peak around ~2015 with a steady decline through to now. LLM's came to market in their current form in the last couple years, and took a couple years to be broadly adopted. There was a clear and obvious market fall off way before AI / LLM.

> Reddit isn't comparable

I agree with that in isolation; but since I don't agree with the AI premise this isn't especially relevant. I don't think AI will replace Reddit, I think one of the other major platforms will absorb it's users like Reddit / Hackerrank / better documentation / back searching absorbed SO's users through 2015-2021