Except they had an agreement for 50 years to keep it that way. So basically what you mean is anyone can change their mind, which means agreements aren’t worth the paper they’re printed on.
Then why would anyone agree to anything?
Then why would anyone agree to anything?
Pretty much. They are only as effective as the body trying to enforce it. The entire point of being a sovereign nation is nobody can force you to do anything. Now it is in a nation's self interest to not violate agreements and get along nicely, but sometimes the calculus changes and the punishment may not outweigh the benefits.
It would have been better for Hong Kongers if they’d kept it, but alas here we are.
The only thing you can do about it is shaming them, sanctioning them, going to war if you really care, ...
(some obscure movie quote, probably Mark Twain or Lincoln)