If BitFury resolved a single doublespend transaction differently from the "main" blockchain, people would(/should) realize that it now has the power to cancel transactions, and stop trusting bitcoin ASAP. At that point you can no longer trust the blockchain, since you can no longer know you're looking at the "true" blockchain, and not a fake one that has been presented to you to make you do something (e.g. pay someone real money).
In any reasonable person's version of "wait X blocks for confirmation" (currently mostly 3), X would be infinite.
they make money by mining bitcoin, if the value drops then their return, even if they sell all BTC immediately, would be affected. They just spent $100m on a new mining facility so to trash bitcoin would be to write off that investment. They cannot make good money by screwing over bitcoin without screwing themselves over.
Best case scenario is that at the end of the effective life of this DC BitFury decide to doublespend their coins to allow them to spend all bitcoins once, however by that time their relative % of the network will be less than 50%.
The remaining American Bitcoin traders are certainly gullible enough to keep buying and trading a 51%-compromised coin. (I mean, there are people who still think Paycoin could make a comeback.) But American traders are a sideshow - all the action is in China (miners, actual traders). So the question would be: will Chinese speculators keep gambling on a 51%-compromised coin?
(And of course the MMM ponzi buyers, whose judgement is sufficiently bad that they wouldn't even understand the problem.)
