I don't see the practical issue with a model being brittle in the face of imminent mission failure. The model breaking down shortly before you self-destruct the whole thing seems like a rather minor concern. It's entirely irrelevant at that point what the model is.
It turns into an issue when somebody throws the software into a new environment without looking at it or it's requirements and then doesn't do any testing with it. But that's not on the original developers. Their solution was entirely valid for their problem.
Even if they had done something like report the maximum value instead, the rest of the software for the Ariane 5 could well have been expecting it to do something else entirely which would still result in a serious problem.
It's an issue of inappropriately using software in a new situation. Without knowing and account for how it behaves, you can't just use it and expect everything to work perfectly the first time around. It doesn't matter how well the software accounts for various issues - at some point something won't have only a single correct answer and the software you are using will have to pick how to behave. If you aren't paying attention to that, it can/will come back to bite you.