Seriously? Are you sure they are worst "wasters" of energy on the planet right now? What about cars, and eating meat, and Christmas lights, and so on? Have you checked your own energy consumption today?
The argument made was that time travelers would go through the effort to punish miners, of all people.
There is a lot of other senseless wasting of energy going on theses days.
It's pretty hard to objectively judge what's a waste of energy.
My biggest concern is that a lot of crypto mining was and still is coal powered. I'd probably care less if it was all solar powered.
I don't like if they burn coal for it, but there are also other energy sources, sometimes there is even spare energy that needs to be used somehow.