Question about time travel (terminator series)?

Okay, so in terminator 4 john conner had to ensure that carl reese survived so that he would eventually time-travel back to the 80's and save his mother. But what if carl reese was killed, or otherwise unable to go back in time? What would happen to John Conner? Would he just cease to exist? I think that nothing would happen to him, because he is already there, so how could he just disappear? He may not exist in another plane, but he would exist at the time and place he is in (2018), right?