Bitcoin's protocol doesn't just rely on the idea that no one will get 51% of hashpower to avoid getting attacked, the point of PoW is to make attacks so expensive that it would deter attackers from doing it. So, theoretically miners could try it, but it would be hard to profit from it, and it won't be sustainable in the long run. Also, from the point of view of miners, it's like killing your goose that lays golden eggs - if their attack is successful, it would cause panic and drop the price, thus reducing their profits from mining.
that only considers bitcoin's mining incentives. the real danger of highly concentrated mining inside china's borders are incentives that are
external to the bitcoin protocol.
here is one possible motive for such an attack: the chinese government forcing miners and mining pools to attack the network under threat of imprisonment.
now you might say this is an unlikely scenario, and maybe you're right, but it exemplifies the problem of thinking about this issue purely in terms of the bitcoin protocol. if the chinese government wanted to crush confidence in bitcoin this is one possible option---and since they can leverage the capital of miners (instead of their own) they can do it at virtually no cost.
This answer is closer to the point. In a time of WAR, the Chinese miners would be funded by a war effort to destroy Bitcoin.