[beaner]

This isn't really true. The only thing you can do with 50% of the hashpower is determine the most recent block. And it still has to be a legitimate block. You can't undo history very far. And to the extent that you can, transactions either go through as intended or get reversed, and the owners keep their money. Money doesn't get redirected.

Any blockchain going through repeated attack would be pointless to use anyway, so anyone with such hashpower has an incentive to cooperate in the long-term. It's also not possible to stop others from contributing hashpower, so you don't really own it in a meaningful sense.

[kurtisc]

It would be possible to fork to create a higher limit with that hashpower. It would also be possible to have 51% without making it publicly known. But nobody needs to bother with that when they can just double spend.

[beaner]

A fork can be made with any amount of hash power, it doesn't need to be the majority. A fork happening doesn't mean it will be followed, or even contentious.

51% attacks do not allow double spending on the blockchain. If a block is overwritten the attacker may choose to change the destination of their own transactions for that block, but the previous spends are undone. This is why merchants wait for multiple confirmations. Typically 6, which would require a 98% attack to undo.

At this point it'd take a nation-state to produce that much of bitcoin's hashpower, if it's even still possible.