> 1. The value of bitcoin would fall to basically 0. The reason bitcoin has any value at all really has to do with trust - belief that the protocol is secure and can't be broken. In this case, an easily detectable spending "override" would cause faith in the protocol to evaporate.

That seems a little out there.

(1) There is something like 1 person who uses bitcoin to move billions of dollars around. The small-time users (<$1,000,000 per transaction) wouldn't have any reason to lose trust in the system. If some company loses $1 bil in dollars, I really don't care; I don't live in a world where it affects me personally. Nobody is ever going to put serious resources into stealing from me because I'm just not that important.

(2) If they re-write the block, that doesn't mean that they've gotten away with it. Most countries have a legal system that would sit up and take notice if someone signs up for a billion dollar transaction, takes the goods and welches on paying.