I had a conversation with a friend this afternoon on the same theme as earlier, but with a twist. A similar challenge came up, essentially, "We know that either there is no time travel, (or else the time travel thing does cause time-line splits or other shenanigans) because Hitler was not assassinated, strangled as a baby, taught pacifism, etc."
I think, though that this still isn't counter-proof. After all, how many people today would go back in time to assassinate Napoleon Bonaparte, Genghis Khan, or Oliver Cromwell? Sure, they were bad guys, but traveling back in time to kill them just seems somehow excessive at this remove.
And as bad as they were, each in their own way helped pave the way for the world we're living in now. It won't take that much time until people think of Adolf Hitler as just another jerk.