Excellent post. Agree with all major points.

I think Less Wrong experienced the reverse of the evaporative cooling EY feared, where people gradually left the arena as the proportional number of critics in the stands grew ever larger.

I'd think it was primarily not the proportional number of critics, but lower quality of criticism and great users getting tired of replying to/downvoting it. Most of the old crowd of lesswrongers welcomed well thought out criticism, but when people on the other side of an inferential distance gap try to imitate those high-criticism norms it is annoying to deal with, so they end up leaving. Especially if the lower quality users are loud and more willing to use downvotes as punishment for things they don't understand.