And if PRC had simply taken the NT at end of lease, and not reached an agreement on the other areas, what would UK have done?
I'm just trying to make the point that had an empire still been in control of HK, things would have looked very different, but that's not the case so they don't. There's nothing inherently with HK's connection to mainland that makes it impossible for it to be self-sustaining, but there was never a point, or an economical or political will to do so.
The most obvious comparison to HK is Singapore, which is now a free state and has its own military, together with being one of the top financial spots in Asia. Yet the history is not that different from Hong Kong.
The different outcomes are not down to the simple fact that HK Island is a barren rock that would be technically impossible to sustain independently, but a complex result of geopolitics which today makes it impossible to release from the grasp of China.
> All British government administration of Hong Kong, terminated at the moment of the handover in 1997.
You answer your own question. The handover wasn't, but is a legally binding agreement with the United Kingdom, where among other things the "One country, two systems" principle was agreed to be in effect until 2047.
> And let’s be honest here, when the British signed the 99-year lease, they never really expected to leave Hong Kong.
The 99-year lease was only in regards to the New Territories. The Treaty of Nanking stipulates that HK Island was ceded "in perpetuity" to the UK. There's also the 1860 Convention of Peking. The PRC did not "recognize" this during negotiations, but as we can all see it seems par for the course for the PRC to simply "recognize" whatever parts they agree with at the time.
If one is to draw the purely legal argument here, at least be consistent.