All engineered products are designed to be good enough. Like you mentioned you can't design a car to be arson proof, EVs today are designed to be crash safe up to a certain limit. It is likely they design the battery enclosure to be safer than the passenger cabins so the passengers and driver would be dead before the fire can start from a crash.I often wonder, didn't the engineers designing the EVs and their batteries take into consideration the problems with putting out fires? Yes ICE cars burn too, but not with the intensity, pollution, or the higher risk to the people that have to put out the fires. Flame suit on, pun/no pun intended.
However, you can only design enough redundancy for manufacturing and design defects. You can QA and QC manufacturing issues but if you have a design problem you obviously won't know until the real world put enough miles on it, and by the time you have enough data you likely had obsoleted the design for something better.
Gas engines aren't reliable on day 1, and gas engines have like 100 years of head start. I think EV will be better and we will have safer and cheaper batteries for most cars eventually, once we reach certain good enough range. I wish LFP and solid state would get us there for the commodity cars and the high performance, not as safe, batteries would be only for sports cars by then (and the owners know what they are signing up for).