I think part of the problem with believing that "at some undetermined point in the future, it will improve," is not a compelling argument against turning off crypto-currencies. From an emissions perspective Ethereum has been clinging to the belief that Proof-of-Stake is better, in theory, and is perennially, "just around the corner." In the mean time it has been and continues to run on Proof-of-Work and contributes in no small part to the environmental impact these crypto-coins have.

Internal combustion engines are already really close to their peak efficiency vs emissions output and they still contribute a huge and growing amount to overall CO2 emissions. We need to design cities to curb the need for cars, replace as many as we can with electric, and design better, more accessible public transportation. Not seek more efficient internal combustion engines and promote their use.

Crypto-currencies account for an abysmally small number of financial transactions in the world. Literally inconsequential. And they chewed up an astounding amount of energy in order to process them. To scale that up to a world where everyone buys their bread and milk and candy bars with crypto-coins is... well not feasible would be the polite way to put it if you still understand physics and the laws of thermodynamics.

I think there are plenty of other reasons to ban crypto-currencies when we get to regulation and protecting consumers.

It wouldn't take much for the governments of the world to shut the whole enterprise down and I think they should.