Crypto currencies are a passing fad as ultimately all governments will eventually ban using them.
To fail to do the above will totally undermine a Countries economy. Imagine a situation where no tax was being collected because everyone was using invisible Crypto currencies.
But part of the reason they work and are popular is that they're unbanable, you'd have to turn off the internet to stop some of them.
For those mining with just one gpu to make a bit of money for Volta have you actually trying to convert crypto to real money? It's not that straightforward. Unless you consider taking selfies with your passport, sending sensitive personal information to some Taiwan based company that keeps telling you your passport photo is not clear enough and waiting weeks only for your bank to decline the transfer as straightforward...
Gemini, owned by the Winklevoss twins who started Facebook with Zuckerberg, turns your bitcoin into dollars and sends it to your bank.
Coinbase, nasty fees but used by millions.
There are others but they're two of the most reputable.
What bothers me is the environmental impact of mining, I'm sure there are plenty of illegal farms tapping into illegal power supplies, millions of gpu sold that would've never been sold, a bit like illegal cannabis farms and even if they were paying for the power, most electricity is non renewable and reliant on dirty fuel to provide the electricity.
With cryptomining you make more money the less your electricity costs, so in the west where electricity prices are higher it's spurring green energy spending, Solar in particular has gotten a huge boost.