What is a term/phrase for an event that never occurred, as it was dealt with before it happened, then the original event seriousness is questioned?

What is a term/phrase to describe a potentially serious event where prior mitigation has resulted in either nothing, or little, happening, causing people to question the actual seriousness of the original event? eg Y2K bug - $300 billion meant it never happened, and it's now considered a bit of a joke, or COVID-19 lockdowns resulting in no viral transmission in a community, and people then questioning whether there was actually a need for the lockdown.

This doesn't answer your question directly, but the concept you described seems related to "survivorship bias."

Survivorship bias or survival bias is the logical error of concentrating on the people or things that made it past some selection process and overlooking those that did not, typically because of their lack of visibility. This can lead to some false conclusions in several different ways. It is a form of selection bias.

https://en.wikipedia.org/wiki/Survivorship_bias