Let's say the world's banking is running on blockchain. The US Congress is basically a veto on world banking, and I don't see how blockchain technologies can change any of that. The US Congress can pass a law saying any US bank and anyone transacting with them must use a specific version of the blockchain code, decided by the Department of Commerce.

If it's illegal to transact with the US under a previous version of this hypothetical blockchain banking technology, that's your 51% right there.

How could blockchain allow legal banking institutions to refuse to follow the law?