It's actually not. If the energy used to mine is sustainably generated, there's nothing BTC can do to hurt the environment.

IMO the fundamental problem with all (PoW) cryptos is the assumption that computing power is distributed fairly when it's clearly not. Building microchips is not easy, it would not be hard for a government who wanted to to amass >50% of its nation's compute power.

And PoS assumes that you'll never end up in a situation where a small number of users own the vast majority of the wealth...(lol)...and that even if they could they would be disincentivized from destroying their own wealth. But the ability to devalue a society's currency has immense value, especially if you're powerful enough to be positioned to fill the vacuum with something of your own design.

And the last holdout argument is that "the network belongs to the people, if the people don't like certain behavior on their network, they can fork a version without it". Except that no lay-person will ever make a decision like this. Virtually every single crypto miner downloads someone else's code and runs it without question. You can't have accountability in a distributed system if it relies on intelligence and integrity that doesn't exist (or the cost of which is unreasonably high).

The environmental cost is a red herring, but that doesn't mean the technology isn't fundamentally flawed as a currency. Maybe for other, more technical applications, but not currency.

Edit: consider the situation where I hook my own solar panel to a PC and just mine Bitcoin with it. Does this hurt the environment? Of course not. Is it a good use of the power being generated? Of course not. Do people have any reason to be outraged at me doing this pointless thing? Of course not. That's my point.

Now if I hooked a gas generator up to the PC, that would be another story.