I'm sure the price of time travel was at the cost of the user's own life (or probably more that we don't know about yet). Some kind of restriction that I'm sure we'll find out in 2.0.

Warping Darnus or Bahamut would have been like wiping his hands of them and sending them to a new generation of people to deal with altogether, i.e. just postponing the genocide lol.

He also sent us into some unknown place where we lay dormant until 5 years have passed...that is, he didn't shoot us straight into the future.