Since the malware writer would not have any expenses, any amount of Bitcoin or any cryptocurrency mined on that botnet would be profit. The electricity cost is zero for the attacker, and with some good social and software engineering, the virus could be spread very far. Also, the botnet of thousands of computers can still be rented out for blackhat purposes. It doesn't necessarily need to be dedicated to one thing, it could be multipurpose. This is kind of like this guy: https://www.reddit.com/r/IAmA/comments/sq7cy/iama_a_malware_coder_and_botnet_operator_ama/. He made malware which mined bitcoin, was also a banking trojan, and could ddos.