Airlines do try to cancel follow-on sectors if you miss one. How well this works depends on the airlines involved. Technically you must reissue the ticket and pay the change fee to skip a sector. Sometimes a sector can be skipped, accidentally or on purpose, but it is against the fare rule. Again enforcement varies with the airlines involved. AA has the best computer system for enforcing this, and CX works well with AA, so chances are that it will be automatically canceled and CX won't let you fly without a reissue. However mistakes do happen, so the results vary randomly. Depends how much time you have for a reissue, or whether you care if you miss the SFO-HKG flight by a day.