You're not entirely wrong, but I do think it's a gigantic oversimplification to only look at the value of Hong Kong in terms of percentage of GDP. While I do agree that a crackdown is likely, I also think the consequences to China's economy, both from direct factors but also from indirect factors that are hard to predict, may be significant in doing so.
At the end of the day, it's my opinion that this struggle is from the CCP side of things not a rational one. They are willing to sacrifice "rational" values for the sake of things like honor and saving face, and I think (and to some extent, hope) that there is a risk that type of decision making will cause a downward spiral for the regime.
I can't remember who made this point, but there's a historical anecdote about how "totalitarian regimes are suicidal". Sooner or later the fascist cocktail that has fueled economic growth explodes into unbearable oppression, unsustainable expansion, economically questionable decisions, or all of the above. I've heard the point many times that just like the Roman Empire was always just "one missed grain shipment away from a revolution", the only thing really standing between the CCP and a popular revolt is the downturn of the economy. The only reason people are tolerating the suffocating oppression of the state is because of tangible rewards.