If you view software as a bunch of contracts between different services/libraries/etc. it is hard to ensure that each piece upholds its contract, so we trust them to a degree. I can see how a failure in one place due to load would be hard to catch or gracefully fail from.

That said they have a pretty explicit cap on maximum activity (number of planes) so it is weird they didn't test around this. It isn't like 400,000 aircraft suddenly DDOS their system.