The constant conundrum of time travel films can be summarized succinctly — if someone changes the past in any way, then that action will have ripple effects which completely change the future; what happens to the future when you change it out of existence? That's the essential paradox of time travel, and it's just as complicated in a multiverse with free will as it is in one deterministic linear universe. Time travel is thus more complicated than most people think.
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