PoW challenges that make bots not viable.

You mean a la Anubis? But people also seem unhappy with that; and in any case Anubis is designed to stop ai crawlers; it doesn't work against a targeted crawler or a targeted dos attack.

People are unhappy with Anubis because it's not designed to stop "AI crawlers", despite marketing as such. It's designed to stop DDoS attacks on layer 7. Anyone who pays the computing-fee gets to pass, regardless of species.