doesn't seem "overgrown" to me. It actually seems dried out and dying

I think both. There's more post and comment volume than I remember from "the good old days", but it's much lower quality in terms of density of interesting contributions.

because the culture is so negative people don't want to post here.

I don't agree with that as the main cause. I don't think it's a negative culture, so much as a lack of positive culture. The people doing interesting work on rationality, decisionmaking, and AI are mostly not doing it here anymore.