Except now the botnets need a lot more computation and thus electrical power to generate the same amount of traffic as before, making DDoS attacks more expensiv

Except now the botnets need a lot more computation and thus electrical power to generate the same amount of traffic as before, making DDoS attacks more expensive for the attacker, and less expensive for the service. I think there is a second cost aspect to this as well; right now, hacking low-powered IoT devices and making them part of your botnet is (relatively) easy and valuable, but as their computing power is quite limited, a PoW defense should make them less viable for DDoS attacks, decreasing the amount of free attack power.