The Vex would've been far more threatening if they all had individual minds that were connected telepathically/technopathically? That'd make them hella powerful.
Also, time travel never makes sense. I can deal with teleportation but not timetravel.
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Just remember paradox theory. Going back to change something would remove your reason to change it. Two things happen, that instance of you is sent along another time stream with the changed effect, while in the old one you never existed.