Well, the fact that the majority of the mining pool runs out of China certainly represents a counterparty risk. I'm not sure why folks regularly ignore the 51% issue.
While there is certainly some amount of quantifiable risk given how centralized mining is in China, each individual mining actor is still incentivised not to participate in a 51% double spend attack as the value of bitcoin would rapidly fall towards zero if a double spend attack ever did occur, making their ASIC investments unprofitable in a hurry.
I don't know. It's like nukes. They say, no one would use them, because that would let the cat out of the box. But... on the other hand, what would happen if someone used only a single nuke? Could we get to the circumstance that Bitcoin was "too large to fail" and a someone could get away with a double spend?
I would like to know the same, but I do have a theory. What will they do? If they start making invalid decisions, the rest of the network will fork. It won't be pretty, but btc will survive... Or is there a better explanation of risks involved?