When that technology becomes a non-trivial part of the global CO2 emission without bringing any value for anyone except select few "investors", it becomes an issue.

Completely shutting down all cryptos would have the following effects:

- Immediately reducing electricity consumption worldwide

- Making money transfer for illegal activities (drug, human trafficking, prostitution, gambling, ...) harder

- Making money laundering of said activities harder

- Reducing the electronic chip shortage

- Forcing a bunch of young, adventurous, often quite clever people to find another source of revenues. Hopefully by providing some tangible value to society

It sound like a good plan. The only real I see are for people using cryptos in authoritative regimes, so overall a net benefit for the world.