It will harm the environment, but no less than the rest of our economy. There are two problems being conflated here. In basically all of society, people harm the environment in order to acquire resources. In proof of work, you go one step further and proceed to destroy the resources you just acquired. This is because destroying resources via reversing hashes is a costly signal that cannot be faked and is hence not susceptible to a Sybil attack.
Proof of Stake doesn't have this problem. The mere act of owning a coin does not entail the destruction of value. In proof of stake, you just make money, probably harming the environment along the way as you do, and buy the coin. That's it. The resources you obtained are not destroyed, they are passed on to whoever you bought the coins from.
In both proof of work and proof of stake people are doing work (using energy) in order to obtain money.
In proof of work you work to obtain mining equipment and electricity and then use that to perform the work of securing the network (nothing is “destroyed”). You are then paid for doing that work. The more work you do relative to other miners, the more you will get paid.
In proof of stake you work to obtain eth (for example) which then grants you the privilege to run validators. The more you stake, the more validating you can do. You are paid money for successfully validating, and charged money for failing to validate. You will need to do more work to replenish money spent on unsuccessful validation. Ultimately, the more work you do relative to other stakers, the more you will get paid.
It’s going to be a little more difficult than it is for proof of work to measure how much total energy is used by proof of stake, but it still should be doable. Maybe it will be less than the energy used by proof of work, but maybe not.
