Not necessarily disagreeing, just unaware, how does HK need China? They were doing quite well for themselves when they were still part of the UK, and independent Asian city-states already have a proven record as also seen in Singapore. (Unless you're saying simply that China doesn't "need" HK at all, in which case I agree.)
HK needs China, at the most basic level, for resources. We import almost all of our water, food, and power from China.
HK doesn't need to worry about military or political protection so we can discount it as needing China for that.
But what you're looking at, should HK separatists get what they want, is a radical change in the economy, with basic life stuffs becoming extremely unaffordable for the average person.
The average salary in HK is something like 15k HKD (or roughly 1.9k USD), while the median is 15,500. But rent is comparable to the Bay Area in the heart of the city, and your basic 200sqft flat will do you in 1k usd a month or more. Presumably if China cuts ties with HK, it cuts down on mainland Chinese businessmen inflating our property market as a way to launder money, but not by much.
With a population of 7 million, it takes little imagination to picture what happens if we have to start importing our resources from countries other than China. (It gets very expensive.) Essentially, tons of people fall into relative poverty, unable to afford basic needs. This is worst case.
This is a bottom-heavy economy with extremely toxic distribution of wealth. Crony capitalism is king in HK, and all the wealth is concentrated in land owner tycoons who have wink-wink deals with the government and contribute nothing back to society. There is no such thing as "trickle down" in Hong Kong. This is a first world hyper-capitalist city with no significant safety nets, too, outside of functionally free healthcare.
The only mitigating factor is that the HK government has absurd amounts of surplus money that it can spend.