Well for one you can't change the past (barring the specific breaking of that rule), thats a law of magic, and one that has been specifically stated to apply to genie wishes (see OUATiW, which I believe also is the first time they explicitly spelled out the laws), this is also why Emma was sent to the WR, changing the past to make it so Emma isn't the Savior is impossible so it does the next best thing, creates a reality where Emma Swan was never the savior.

As for my assumption, while my explanation does not have any root in canon, neither does yours, and mine does not contradict anything, while yours does.

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