Even if China controlls 80% of the mining power that doesn't mean that we should fear a 51% attack. Do you actually think that the guys who control mining, being Chinese or not want a 51% attack??? of course not, otherwise their investment would be worth 0....
Come the halvening, their investment will be worth 0 (unless other miners stop mining/BTC price doubles), so not that improbable :-
Once the halving takes place we will see a lot of old miners getting of the boat. It wioll be uprofitable to mine for most of the oldest mining hardware. Since a lot of people will stop mining with their old hardware the most efficient miners will still be profitable!