I don't think that China has 90% of the hash rate though
This
site puts the figure at approximately 81%, regardless of which is totally accurate, the majority of hashrate is concentrated in China, especially when compared with other countries;
Those are the pool's hashrates, not the hashrate that is actually located in the country.
Do you honestly think the Czech Republic has more mining gear that the Rest of the world - China combined?
If 'China' (you probably mean the chinese government) wants to 'takeover Bitcoin' (whatever that means...) they will quickly learn, that this operation will cost them billions of USD and the result will most likely be another shitcoin fork instead of a 'takeover'.
Billions for what?
They can just seize mining equipment from all the miners in the country, seize Bitmain and Cannan, and simply use their facilities to produce gear at 1/4 of the cost you as a private miner in the US and EU have to pay for it.
And billions, what are billions for China if they see this as a threat? They offered trillions till now in bribes and so-called loans, what would be 2,5 billions worth of mining gear if indeed they see
BTC as something that might undermine their position?
Satoshi had figured it out that really early:
The incentive may help encourage nodes to stay honest. If a greedy attacker is able to
assemble more CPU power than all the honest nodes, he would have to choose between using it
to defraud people by stealing back his payments, or using it to generate new coins. He ought to
find it more profitable to play by the rules, such rules that favour him with more new coins than
everyone else combined, than to undermine the system and the validity of his own wealth.
Satoshi was talking about a hacker, a single person looking for profit, here we're talking about a country that would have no other purpose than to eliminate decentralized cryptos.
Fortunately, their dominance is crumbling away and if they ever plan on doing this (which would be perceived by other countries as economical war) it might be too late.