I think that the president of PRC has a better chance to harm Bitcoin.
Please correct me if I am wrong (and I mean it, cause I am not 100% certain of this).
So, imho, if China decides to shutdown their internet or disconnect it from the rest of the world, then most Bitcoin miners will be cut off.
Until the network will adjust for that it takes quite a number of days - I don't know how it's calculated - but for some while the transactions will be VERY slow.
If this will take too long (which is a relative term) this would harm the network a lot and some may / will leave it.
Now, after the difficulty dropped this much, the Chinese have a chance that at the point their network is back to even do a 51% attack. Though unlikely since it's bad for business, this still can happen, theoretically.
Of course, I may be in a big confusion, somebody should check this theory against the possible reality.
Ok quick back of the envelope calculation for you. Let's assume China has 60% of global hashing power (as stated here
https://www.buybitcoinworldwide.com/kb/which-countries-mine-export-most-bitcoins/ ).
If those 60% were to go offline at the exact same time, the remaining 40% miners would indeed take longer to confirm blocks. A block that took an average 10 min before would then take exactly 10min/0.4=25min to confirm.
This state would persist until the next difficulty change which will be an expected 7 days (or a theoretical maximum of 14 days) away. After the difficulty change, block confirmation time would be back to an average of 10min.
The impact of this scenario is therefore minor.
As to the 51% scenario, I am not sure how one actor would be able to control all of China's mining resources, even if they are backed by the government.