Without saying too much, let me put it this way: An intelligent botnet owner will not abuse their bots. For example, they would mine on a quarter of the CPU, or half, until the system was idle. Then they would use all of the CPU. Users who have slow computers either run malware scans or reinstall, and if they can't, they find someone who can. Happy bots means a happy botnet owner.
What you say is true, but most computers these days shut off when they're idle. The default settings in Windows shut off after an hour or something, and most users never change default settings (blinking 12 effect). And mining on a quarter or half of a CPU when not idle means the botnet has just become that much smaller.
But the issue is more fundamental. If owners of insecure computers have higher electricity bills, they will be incentivized to clean up their computers, or to buy a service to do that for them, or buy computers with a lower risk of being infected. The market will sort this out. Non-consumptive botnets are worse, because they give no real incentive to the only person with a real ability to do anything about it, the computer owner. All the costs are borne by others.
