Pro: Stop with your logical thinking and get comic booky here.
The laws of physics that protect a temporally displaced Superman's place in history are identical to the movie physics that allow Marty McFly travel into the future to do something about his kids.
But allow my friends on the Satellite o' Love to help you out here.
You should really just relax.
History will be RESTORED when he's deposited back at the point he left, thereby being able to create the future history that the Legionaires know.
Had an inspired thought. We're viewing the events of Argo
as they transpired after
Superman returned to the 21st Century.
We're seeing the events after the ripple effect restored
the future around Supes and the Legion.
The events as they originally transpired in the future are an altered timeline where Superman was at best a minor historical figure. Once he returned home well... the future changed back and he was THE most important historical figure in the history of the multiverse.
I really need to figure out a non-confusing way of articulating time travel theories.