If the US restricts or bans CX flying, HK would do the same to US carriers.
That's a false statement in this hypothetical scenario. CX may be the flag carrier of Hong Kong, but it is NOT a government owned airline. The US would not be banning every single carrier from Hong Kong, it would be banning CX ONLY. Therefore the government of HKG would have no authority to ban every US carrier in a tit for tat reaction.
The odds of the above happening is highly unlikely so it's all a moot point.