The Terminator movies feature a time machine built in the future, that skynet keeps using to change it’s fate. So it never loses, even after the resistance forces have won, destroyed it’s central core, it is able to send back in time, a T-800 to destroy the leader of the resistance, John Connor. However, the resistance is able to send back in time a savior for John, Kyle Reese. So the final battle does not take place in the future, but in the past, here in LA 1984.