In addition to the tight integration which would be impossible to untangle, the territories which were not due to be relinquished were too small to survive independently:
As late as the 1970s, it was taken for granted that the expiry of the lease would be no more than a technicality. But Peking made it clear that it expected the return of the New Territories to Chinese control. The British had always accepted that, on their own, Kowloon and Hong Kong Island were too small to be self-sustaining. So, after initial resistance from the Prime Minister, Margaret Thatcher, it was accepted that sovereignty over the entire territory would revert to China in 1997.
Independent newspaper, March 1993