A 51% attack is not just a mining pool getting 51%.  If some entity got 51% and mined secretly all past transactions could be wiped out from the block they started mining.  It could mean weeks or months of transactions could be wiped out.  If someone wiped out the last 6 months of Bitcoin transactions I think that would kill it.  The cost analysis that I have seen calculated a price based on buying equipment from vendors.  A 51% attack like this would mean the entity would make their own hardware.  If they had control of places that manufacture chips that would make a big difference with the cost.  I don't think it would happen but it is not completely out of the question.