While I agree with you, especially with the benefit of the hindsight, I used to spend many CPU cycles on SETI, looking for freaking aliens.

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.

It seems to work well enough for at least some use cases, no matter how low the number of transactions. Further developments like Lightning Network are being worked on to enable other use cases.

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.