China used Hong Kong as a neutral trading ground when it was closed off (to sell things and import things), which is why much of China's industrial capacity was built up in the Pearl Delta region. If China closed up again, they could be using Hong Kong again like they did in the past, especially if they keep it separate from the rest of mainland administration like they do today. But it isn't guaranteed.

Times change. A typical Chinese metropolis completely dwarfs hong Kong. That's why China was willing to back pedal on the autonomy deal for hong Kong because its no longer means much as an economic powerhouse. Almost every other Chinese city has left it in the dust.

You know this, but a lot of people are just behind on the current economic reality. Not surprising given the speed at which china moves.