I think the best solution would be to properly tax carbon. That way Bitcoin miners would either become unprofitable or move to greener energy.

I don't think it's a good idea to establish the precedent that gov't can decide what you can and cannot do with your energy. You may think it's a waste of energy, but if the externality is properly taxed, I don't see the problem with letting it continue

If you destroy Bitcoin, another currency would take its place.

Some have tried, they have all failed. Bitcoin is international. A 51% attack is so implausibly expensive that nobody really has the resources to pull it off. Even if you had enough money and energy to burn, there is the small problem of acquiring enough of the specialized hardware to do it (ASIC miners), and potentially the specs and fab to make that hardware. People will see it coming a mile away. Don't want to use ASICs? Enjoy at least a 100x increase in energy and equipment costs. And it gets more expensive every year. If you had that much money to put into destroying Bitcoin, it would be much better spent on an ad campaign telling people Bitcoin was bad than doing a 51% attack.

A 51% attack doesn't prove Bitcoin is broken, it proves the protocol is working exactly as expected. A 51% attack causes a temporary fork. This happens all the time organically when two miners find the next block at the same time, it's a natural part of the protocol. That's why for really large or important transactions on main chain, you wait a few blocks before considering them fully secured.

Bitcoin's value to society is the ability to easily transfer money from point A to B and having a clear fiscal policy it has kept to for 15 years, 365 days a year, 24/7 without a single hour of downtime, a bank holiday, or getting hacked. There's a reason big money like hedge funds and private banking are investing in it: it's actually useful and has massive potential. The market cap of Bitcoin is 850 BILLION USD, that's bigger than the GDP of Sweden or Israel or Vietnam. People use it to move over a trillion dollars of value a year. You can debate how much of that movement is trading & speculation vs use as a currency, but it's a trillion nonetheless. I personally pay for things regularly with Bitcoin, you'd be surprised how many places you can spend it when you start looking. And it's available to anybody with a cellphone and halfway reliable internet access, including the billions of people who are "unbanked" and lack access to stable banking infrastructure.

Transactions on Bitcoin lightning occur in under a second and cost pennies in fees. That's to send it across the room or across the globe. Remittance services and bank wires use just as much energy and cost 10x-1000x as much. And they waste not just energy but human capital as well, we no longer need humans manually sending bank wires like it's 1910. You just don't see headlines about the energy impact of bank wires or western union because it's not novel, we just accept it as a cost of our financial system.

That's not even getting into the secondary costs to the environment of running a society on an economy based on an inflationary currency which requires that currency be rapidly spent because it's getting constantly devalued. That's a great strategy to rapidly industrialize the world, but it's not a great strategy on a globe with limited resources. Tell me, if you knew your dollar would be worth 10% more next year, would you be more hesitant to spend it? Might you consume less if you knew saving money in your bank account would actually cause it's value to stay the same or increase over time? Might you focus your spending more on quality products that will last instead of just buying the cheapest thing because if it breaks, you can just buy a new one? This isn't just on a personal level, this same kind of calculus is used by big investment firms to build everything that won't last. Buildings, stadiums, entire cities, financed with money that is constantly losing value. Bitcoin's value relative to goods and services will fluctuate like any currency does, but the supply of the currency does not increase. There are 21 million which will ever be minted. Your 0.1BTC will always be 0.1BTC and will always represent 0.1/128M% of the total supply. If the Bitcoin economy grows, you share in that growth and the value it produces instead of seeing the difference printed away and given to whoever controls the money supply and whoever they want to give it to.