Give an inch and they'll take a mile. The slippery slope is a very valid concern with one of the most powerful potential adversaries actively wanting to integrate you and dismantle your system of government.
From their perspective it's not hard to imagine China trumping up charges (or getting one of the other listed nations to do it) to get their hands on someone they want. Likewise, having the law on the books at all makes it a (comparatively) very small step to simply add more things to the list of acceptable extradition reasons.
China's domestic actions have not given HKers much reason to trust them, and they know that they won't have any chance of clawing back any measure of sovereignty in their system that they give up now.
China has a lot of vague laws and a history of making up accusations to fit the occasion, with no way for HK judges to examine evidence, and trial with presumption of guilt. This excellent comment on reddit by IosueYu answers the question better than I ever could: https://www.reddit.com/r/HongKong/comments/bvux0z/hong_kong_...
These types of protests are largely encouraged by the West to deepen division within it's enemies, same as with sunni/shiites in the middle east, or gop/democrats by the Russians. You can tell because the West didn't give a single shit when hk wasn't democratically ruled by the Brits, but now democracy there is headline propaganda.
The UK only tried to introduce it after they knew they couldn’t hold on to Hong Kong anymore. So they did it out of spite.
But the UK did try to convince Portugal not to offer the people of Macau citizenship, because they didn’t want to do the same for the people of Hong Kong.
From their perspective it's not hard to imagine China trumping up charges (or getting one of the other listed nations to do it) to get their hands on someone they want. Likewise, having the law on the books at all makes it a (comparatively) very small step to simply add more things to the list of acceptable extradition reasons.
China's domestic actions have not given HKers much reason to trust them, and they know that they won't have any chance of clawing back any measure of sovereignty in their system that they give up now.