You could, but it would likely be against the intent of the contract. Note that it also mentions "or successor body". So a smart contract would still be worse in this scenario.
For this kind of event I don't think it's possible to define what "successor body" means. If the US government falls, is the "New United States" body "US Centre of Disease Control" a successor body? If CDC (and its private keys) gets nuked as part of containment, is the spinoff "US Centre of Zombie Prevention and Control" the successor? Maybe. Depends how the role of CDC was split up in the new world.
Is it an easy question? No. A court will likely have a hard time doing the right interpretation. But with smart contracts it instead becomes impossible to achieve the goals intended by all parties involved in making the contract.
(Of course laws can be enacted and dictate a given interpretation, too, saying e.g. that any contract clause preventing use by the military against zombies is void)
And so it goes for all smart contracts. A smart contract can bring back slavery. But slavery is illegal. Now what?
