I disagree. McCoy went back in time and altered history and thereby altered the timeline. He didn’t create a whole new one. If I went back in time and prevented the Lincoln assassination, I wouldn’t create a new timeline, I would just alter the original one.
Thank you, that’s another point I forgot.
If you went back in time to prevent the Lincoln assassination, what happens to you after it happens? Where did you come from anywhere? Your entire history has changed. And then in 2017, you wouldn’t have any reason to go back in time, hell, you might not even exist. What then?
Time travel actually creating alternate timelines prevents this from being a problem. You now live in the newly created past, from a reality where Lincoln was killed, but having created a timeline where he hasn’t.