There is something I don't understand about this. There are sprinklers inside all of the cabins, did they not work? My husband is a forensic fire investigator and he says something isn't right here. It should have been contained easily to one or two cabins had the sprinkling system been functional. This many turnout to be negligence on Princess's part.
Actually, you seem to have nailed the essential point -- the sprinklers and the smoke detectors are inside the cabins. The fire, unfortunately, was outside of the cabins, so the sprinklers and the smoke detectors were totally irrelevant. The fire probably melted the glass panes in the sliding doors between the cabins and their balconies before enough smoke and heat got into a cabin to set anything off.
This incident may well lead to an amendment to the SOLAS treaty. An explicit requirement for smoke detectors and sprinklers in recesses in the exterior of the vessel, such as stateroom balconies, should be fairly easy to retrofit into existing ships during their next yard periods, so all ships could be brought into compliance within two or three years.