It's not the original timeline because people from the future have arrived. Meaning that things have already progressed past that point in time, and someone is showing up from the future.
All the rest of what is being argued makes perfect sense, but you have to have an original condition where someone in 2018 decides to time travel, or you can't get into the loop.
it also makes the entire series fairly pointless. Neither Skynet nor Kyle would really care about the war, the future, or anything at this point, both would just be time travelling because they have to create John and Skynet. Not so either can win the war, but just so they can go back in time again.
To take another crack at the "closed loop without an origin" argument: a problem in computer programing can create an infinite loop pretty easily, yes? You've gotta have code before that to set up something TO loop though, right? Can loop all you like once you reach that part of the code (the future, in the terminator example), but have to run the code to that point WITHOUT the later parts of the code interfering the first time through.
The first time people from 2018 showed up in 1984, it was, by definition, the 2nd crack at things. You can close the loop right at that second, but logically, you've got to experience things from 1984-2018 once already before someone from 2018 can attempt to change the past. It has to have, well, past once already.