I don't really like what China did with Hong Kong, but some things you surely agree transcend 'contracts' or normative behavior. I can't, for example, agree to be murdered in exchange for money. I might also lie to protect my children from the local murderer, or in any other case where I'd consider the outcomes 'extereme'.

Sovereigns can withdraw from agreements. It's the whole point.

The only thing you can do about it is shaming them, sanctioning them, going to war if you really care, ...

Because the British didn’t have much of a choice.

It would have been better for Hong Kongers if they’d kept it, but alas here we are.

> Agreements aren’t worth the paper they’re printed on.

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.

The agreement was BS imposed by their colonizer. Why would anyone bother to abide by such terms when there are zero consequences for canceling them?