No, it's much worse than that. The nightmare scenario for most people is thinking that a transaction for which they are the recipient did go through and then l

No, it's much worse than that. The nightmare scenario for most people is thinking that a transaction for which they are the recipient did go through and then later have it turn out that it didn't, and the money actually went somewhere else. Because of this, the net effect of a 51% attack would be to very quickly destroy confidence in the system for the duration of the attack (at least).