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.
“Dodged the bullet” is a phrase used in this context.
dodge a bullet or less commonly dodge the bullet
to narrowly avoid an unwelcome, harmful, or disastrous outcome or occurrence coastal towns dodged a bullet when the hurricane veered out to sea
The 20th century geopolitical example is avoiding US-USSR nuclear war and the end of the Cold War, with the conclusion that the Soviets were just pussy-cats in any case.
As far as I know there’s no single word that encompasses all these concepts. Whether the original threat, like the Y2K problem, was overemphasized will always be debated. On the other hand there are all sorts of events that we should have, supposedly, seen coming but failed to prepare for.
The problem with dodging the bullet is that there seems to be an element of luck involved, which may or may not be right in a particular case.