1)You either believe in one determined “block universe” where everything is predetermined... 2) or you know that totality must in some way ultimately be describable by all the possible past light cones, i.e. once you are on a truly fundamental level, the you that throws tails is exactly as real as the one that gets heads, and the difference between those two propagates at most with the speed of light through the model that describes spatial relations (the universe). Here's another, IMHO: 3) Branching does not need to occur as there is no necessary preservation of reality in terms of the Heisenberg Uncertainty Principle. You can imagine a scenario wherein the HUP is broken, but what is the nature of that scenario? Consider that it's not that rules cannot be broken, but enforcement of the rules themselves might be observed as a relative property, dependent upon certain conditions. If not, the MWI is describing infinite worlds to describe a real rules set. Can't we slow light?