It doesn't have a hard requirement on fossil fuels, no. But if Bitcoin is using clean energy, that means it's using clean energy that could otherwise be used fo

It doesn't have a hard requirement on fossil fuels, no. But if Bitcoin is using clean energy, that means it's using clean energy that could otherwise be used for things that provide actual utility - lighting homes, running businesses, charging electric vehicles, etc. Fiat currencies are real because we all agree that they're real. You can pay your taxes with them, you can buy a house or your groceries. I don't think Bitcoin, or other crypto, can yet claim they're real because their worth is expressed almost exclusively in terms of fiat currency. Even if some business does accept crypto, it's expressed as a conversion from an underlying price in terms of fiat.