The thing is that major currencies switching to proof-of-stake just creates a pool of cheap hardware for shitcoins to bootstrap themselves past the threshold of

The thing is that major currencies switching to proof-of-stake just creates a pool of cheap hardware for shitcoins to bootstrap themselves past the threshold of 51% attacks with. So even if every crypto agreed to go proof-of-work tomorrow, it wouldn't change anything - all you've done is "destroy the demand" that is supporting GPU prices, making them paradoxically cheap and creating a paradoxical demand for something else to utilize that capacity. It's another form of Jevon's Paradox in action. The availability of a large supply of a resource "cheaply" will create a demand, someone will find something to do with it. Also, due to the existence of proof-of-stake it becomes impossible to shut down trading of proof-of-work coins, because while you can outlaw trading them on conventional exchanges, it's realistically impossible to stop them from being traded on decentralized exchanges. So if you want to get rid of the carbon emissions from proof-of-work, you have to kill proof-of-stake as well. Just make it entirely illegal to convert any crypto to USD or vice versa, and illegal to interact with any entity who doesn't respect said sanctions. Works pretty well for other financial laws in the US, but it needs to have a sufficiently wide scope. It's not possible to retain "the good crypto" because the good crypto is an enabler for the bad crypto.