The environmental damage from a proof-of-work cryptocurrency is based on market price, so to fix it you need to crash the price. Buying is bad since it bids the price up. Selling is good. Holding is neutral in itself since it doesn’t participate in setting the price, but if we want to make it crash sooner rather than later, then we want holders to sell. A 10% wealth tax on holding would convince legitimate businesses that pay their taxes to sell, so it should help?