You kind of missed my point. If Arnie went back first, even on e minute earlier than Reece, then history woulda been changed, and John would not have existed.
Why?
I think you're overlooking a very simple fact: namely, that by the time the T-800 and Kyle Reese travel back in time (in 2029), the events depicted in
The Terminator have
already happened (in 1984).
That is to say: in 1984, the T-800 and Kyle Reese appeared, one after another, and did what they did.
Then, forty-five years passed.
And then, in 2029, the T-800 and Kyle Reese went back in time, one after another.
Ironically, the best and easiest way for Skynet to change the past and accomplish its goals in this situation would have been to
not send the T-800 back in time. Had Kyle Reese not pursued the T-800 back to 1984, he would never have met Sarah Connor, and would never have become John's father.
Obviously, Skynet must not have known that its attempt to change the past had already failed.
Indeed, the first movie raises an interesting question: is it, in fact, possible to change the past?
By the time you travel back in time and attempt to change things, you will already have succeeded, or failed. But had you succeeded in changing the past, you would never have travelled back in time to change the past. Therefore, you must have failed, and will fail.
Similarly: by the time Skynet sent the T-800 back in time to kill Sarah Connor, the T-800 would have already succeeded or failed in its mission. But had the T-800 succeeded, Skynet would never have sent it back in time: with no John Connor to lead the resistance, there would have been no call to kill his mother. Therefore, the T-800 must have failed, and would fail.
The only way out of this situation, I think, is to somehow know that it's your intervention that caused history to unfold as it did. But that raises another question: could you then, in that situation, choose
not to travel back in time, and change the past?