What part of your post shows evidence of China being able to do a 51% attack, on bitcoin nonetheless? Quantum computers do not exist in this day and age and even Google's claim to have one has been proven to be lacking and not really useful.
Besides, China is different from Chinese miners (who are not necessarily unified as a single team), any news saying China can do 51% is FUD.
The biggest farms are in China. They had already more as 50% many years before... I think, this portion is more at this time.
China isn't a democracy - it is simple - if the state wanted to get some firms under the control of China army - to get they.
But China want to tax bitcoin mining at this time.
The Market was happy.
Then we heard, China want to create a coin for China (to support Chinese interest around our World...). We told - ahhhhh it is only an Altcoin. Not more.
But we know, how difficult to act against a such economy.
China said later - the future is for the crypto-technologies...
The Market was very happy. (+40% in some hours...)
But, if the Chinese coin ready to start, China will be able through (the well detected <= because the tax) miners to act agains Bitcoin Network, and able to support his "Altcoin" in the whole World.
An attack a such Network at this time not a difficult act - it is nearly impossible.
Yes, I know - it is only my opinion at this time (not any article about it at Cointelegraph i.e.) - but for me seems this "simple" steps as a risk already.
Having more than 50% of the hash rate is like being the strongest man alive. It doesn't mean that he will punch you in the face just because he can.
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.
Source:
https://bitcoin.org/bitcoin.pdfAs for Quantum thing, it's still nowhere near ready for mentioned purposes.
Don't worry too much

I should say - I use only my brain - and I have more worry against the change in the climate of our world. Please see the threads in the OP

Maybe a change in the art of mining a good thing then.