No you don't understand, they will literally copy the source code, and go start another chain completely.

Not even a hard fork.

They will just take the Ethereum source code, call it something else, and start their own brand new chain, with a new name, and all the same code without the PoS stuff in it.

Additionally, the reason ETH is expensive is because you can buy it on exchanges,

Thanks having been born just yesterday I had no idea if you traded something the price could go up. Doesn't matter if it's not already traded, if even 1/10th of ETHs eco-system moved over to the new chain it would still be in the top 10 for market-cap.

You sound like the first naysayers to crypto itself, they're always wrong in the end.