There's a valid basis to the idea that the future is hard to change. Most things are the result of many interacting causes rather than a single one. Change one factor and it may alter a few details, but the flow of the current of time is not that easy to redirect. Occasionally a specific event may be pivotal enough that changing it will have a major impact, but there are plenty of other changes that would be damped out by the larger forces of history. This is a concept that's been used in science fiction for decades, going at least as far back as Poul Anderson's Time Patrol series.