Why does the statement "once in a generation" completely obviate any nuance? It makes sense to evaluate things based on events and changes in situation and cont

Why does the statement "once in a generation" completely obviate any nuance? It makes sense to evaluate things based on events and changes in situation and context. It makes sense to not bring up things over and over again in an attempt to change something that people don't want changed. It doesn't make sense to use a phrase as the sole justification for avoiding reevaluating a decision made in a different context. I don't think it's rational to read "once in a generation" as "once in a generation, regardless of any and all circumstances, with no room for any reconsideration ever". Any statement should be considered to be taken in the context that it is given, and constrained by rational, reasonable bounds and circumstances. Assuming anything outside of technical and legal language to mean the most absolute and literal interpretation of the words isn't productive or useful in most cases.