In one of H. Beam Piper’s science fiction stories of ‘alternate timelines’ (a frequent subject of his writing), there is a discussion as to whether an altered event would have little effect, with things canceling out over time and history getting back on an even keel, or make an ever-increasing difference as the changes spread outward from the original point of difference, effecting more and more of reality.
I will admit I believe the first of these; individual actions have little effect on a history largely driven by economics. People simply don’t matter on the largest scale. The names might change in the history books but history would sweep on along pretty much the same path. What could change history? Natural events, certainly—droughts and epidemics and that sort of thing. Again, those are ultimately economic forces (in the broadest sense of the word).
But what if we assume infinite realities? Then every possible outcome exists (if only in potential). History will go on as it did or change radically and everything in between, so there is nothing wrong with a speculative fiction writer imagining any outcome they desire. All should be equally valid. George Washington was assassinated and the United States went on. George Washington was assassinated and the US broke up into warring states. George Washington was assassinated and it prompted an alien invasion. Whatever world we choose to create.
But if we do, we should also pay attention to the underlying economics, to be able to explain why things changed. Even if anything can happen, we should know why!