Wouldn't bot farms just incorporate that as a "cost of doing business" and expand to absorb the computational load? After all, it's not like the bot farmers are paying to add more hardware.
Bot farms exist because they are cheap. You don't need to be perfect, ultimately one needs to adjust the cost of the handshake to ensure that it's higher than the average earning of the farmer.
E.g.: the handshake can be made more expensive choosing a "harder" function for the handshake and giving clients that behaves "good" the possibility to reuse the connections. Bots are penalised because they constantly have to make new handshakes.
But the economic incentives of a botnet are very different from those of a bot farm.
