if cryptos fall back to zero (they have already lost 80% of their value, or $750 billion), their mining will have represented one of the most fantastic waste of energy in modern history. The only "objective" value associated with them is the energy cost of their extraction... Cryptomos will not leave any material traces in the world heritage (because even an abandoned industrial wasteland is a testimony to our past, structural metal can be recycled, etc.).
Cryptos could have just increased the pollution of the planet without ever having been used for anything to protect it.
But taxing crypto-miners is impossible...
This is an interesting point. But I never looked at it this way. Plus, I highly doubt that once in the future waste of energy will be marked as a negative side of the crypto industry. It had a huge impact to supply and demand of the tools needed and it definitely made huge waste and costs for energy, but that will be valuable to statistics in the 1 or 2 years to come, no longer than that. If you look it that way, it is time consuming as well. But that is the price to be paid, we all agreed to it. I don't agree that the cryptos will fall to zero, ever. I believe that is not an option.