Sorry, unless I'm missing something, this is the first time a Tesla battery in a MS has caught fire in over a year after the cars first hit the road, and there have been some pretty impressive crashes of the car to date without any fires. From statistics, there are something like 140,000-150,000 cars that catch fire every year in the U.S. If every time a vehicle caught fire resulted in a reengineering of that car, that would pretty much solve all traffic problems in my area as you'd never see any cars on the road. And the driver of the MS had plenty of time to pull over, shut off the car, get out of the car, and only then did the driver smell smoke.
If the battery pack had been ruptured by the metal object and it took that long to catch fire, I'd say the car held up as well as you can expect without building an impenetrable fortress of steel that weighed 10,000 lbs surrounding the entire car so it would look like an armored tank. There is a cost benefit analysis to any change, and without any evidence that human lives are at greater risk for making any retrofit and without any showing that the retrofit would materially improve safety of the car, I don't see why Tesla would go through substantial expense to retrofit and add weight to the car to ensure that only car per year doesn't catch flame in a freak accident.