From what I understand smart contracts are immutable, so this "bug" can only be fixed by publishing a new contract which is by definition independent from the o

From what I understand smart contracts are immutable, so this "bug" can only be fixed by publishing a new contract which is by definition independent from the old, i.e. unable to access or manipulate any tokens (like these) that belonged to the old one. Additionally, apparently contracts may rely upon one another, and a bunch of the contacts that utilize this WETH token depend upon the existing contract. All of these (and their dependents, recursively) would also need to be updated. I.e. the money is gone unless there is a large concerted effort, but no one should reasonably expect this.