Being the lions’ share geographically means nothing out of context. HK Island is the urban and financial center of Hong Kong, so saying that it would never have survived with NT ceded to the PRC is like saying Manhattan and Brooklyn would never have survived being ceded to the UK because they didn’t hold New Jersey. Sure, it’s impractical to have a hostile mainland neighbor but if the place you’re occupying is one of the worlds largest harbors with all that entails, and you have the military capacity and will to defend it, it’s far from “untenable”. The main reason HK was ceded at all was because of the total decline of the British Empire, it’s not much more complicated than that.

I think there was logistical issues with power plants and water treatment, and such vital infrastructure being largely in the New Territories, is what I remember.

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?

Power plants and water treatment coming from the PRC is a result of history, so one cannot just look at the current state of affairs and conclude that no other outcomes were possible. As with anywhere in the world, the water and electricity comes from where it comes because it's the most economical solution, not because it's the only solution.

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.