~ time travel = myriad civilizations re-building when dictators leave ~
So, if time-travel turns-out to create *alternative* time-lines (a copy of time, up to that point, which you can modify as you please *without* disrupting other time-lines), then wealthy overlords will travel back repeatedly. Why? As soon as they make a mistake in the time-line they choose to dominate, then they will hop *back* in time, to avoid it! That will leave an entire civilization, which had been controlled by a techno-futuristic despot, without a leader or purpose. Over and over, again. *THOSE* time-lines will be far more numerous than our own, original time-line, now. Thus: an entire multi-verse of sci-fi stories await, tracing the collapsed empires of a single time-traveling villain, now long gone! The most important item left behind: an orbiting facility around Earth, with the super-massive “Time Drill” that our villain used to escape!