For monetary transactions that's fine. But they are not the only type of smart contracts. Consider the people trying to e.g. bootstrap systems for using the blockchain as a share ledger for a company for example. There the court will just say "X is the owner of those shares. Ensure the record reflects that." If the system they have chosen for the ledger can't let them rollback a transfer because of a bug, that is the company's problem.

There are a lot of potential circumstances where people will need to find human workarounds for the supposed immutable nature of these blockchains because courts will simply say "this is how it is; make it happen". The above example is "simple": The company can worst case just pass a board resolution to replace the ledger and/or reissue it.

But it is a demonstration that the immutability of the ledger will often be irrelevant, in the face of a court that says "this is the truth now".