True, but why do we need a consistent global state for notes? Unlike Bitcoin, where everyone needs to agree on the exact ordering of blocks in the ledger, why does it matter for notes? Programs should make no such assumption.

I don't follow the key management example. If your key is compromised, why would it matter who broadcasted the first key migration message? In fact it's even likely to be the attacker, when the legitimate owner is not aware that the key was compromised. Isn't this something that can only be resolved on the "social layer", i.e. other accounts helping to determine the correct new npub through IRL verification?