One lives and learns.
I once set a car on fire whilst welding the floorpan (plastic petrol-pipe melted) 2x 2-gallon foam extinguishers proved ineffective but a hosepipe and cardboard sheets solved the problem......water chilled the petrol below it's flashpoint, now sodden cardboard contained any fumes under the "hot-spot"....doors and windows left closed ensured the upholstery ran out of oxygen and self-extinguished!.
The practical issue here, however, is the lack of practicality. hoses that will stretch several thousand miles and supply suficient water at 30,000 feet are a bit thin on the ground 9or in the air)
If water dousing is reckoned practical, then a large tank and a distribution system will be required......Having read many a thread about tankering surplus fuel, I conclude that carting this fire-fighting system around is not cost-effective....just change the batteries for something with lower energy-density but greater inherent stability.
Halon has specific exemptions for usage in certain situations. When it was first outlawed for GENERAL use, the legislators actually thought it through!
AFAIK, Aircraft are one of those exemptions.
Risks with Lipo's are well-known in the model-flying world, but people accept the risk with a MODEL as it is far outweighed by the quietness and extraodinary power offered. Also, of course, instant starts, no messy fuel .smoke,smell and oil-residue. Also, the duration is pretty good and packs can be quickly swapped-over.....the downside is the Russian Roulette of "if/when" will it burst into flames
In commercial Aviation , this is a totally unnecessary risk. IMHO, it's technology for it's own sake.