To take control of bitcoin, you need to take control of 80% if not all the networks and even at that moment, you'd still need a lot of power and money (for the equipment) to be able to pull it off and China's lacking in money already if you're keeping up with the Chinese stuff that's reported by investigative reporters so I don't believe that it's in their agenda to take over the bitcoin network anytime soon, pretty sure that whatever they're doing now, bitcoin or any cryptocurrency has no relation to it or anything like that.
Why would they need 80% and not just 51%?
China has no problem finding the money to build large scale infrastructure projects. I don't see how it wouldn't be possible for them to get 5-10 billion dollars worth of ASIC miners. Also with the current block reward it would only require 32mil USD a day worth of power to produce majority of the hash power. Based on my rough math this would require a 12GW power source and china has a large number of power plants to spread that over. For reference Chinas largest power plant is Three Gorges Dam with 22GW.
51% is smaller than 80% but why only 80% and not 100%, for it to be sure? Hehe. Yes they have a good budget but I think they aren't only interested on it because if they do then they already did it long time ago and there will ever be no banning that will occur in both BTC and BTC mining. 2 to 3 billion dollars and 32 million a day worth of power is a lot of money and all of this is for the hopes of controlling BTC?
But, what can they get after this? To earn more profit? However, I think people will stop supporting BTC once they figured out that it is now fully centralized and China can only get a bad reputation which I'm sure they don't like for it to happen.