Temporal Mutability — AKA The Sliding Scale Of How Easy It Is For Time Travelers To Change The Past, And Why.
Time Travel is one of the richest concepts in Speculative Fiction; altering the past is easily one of the richest Time Travel plots.
Apparently, people (or at least SF writers) in general have a deep-seated dissatisfaction with the past—either their own recent past, or with the whole [...] [tvtropes.org]