The Devastating Truth Of Deadpool 2's Ending
1. So, What Does It Mean?
In order for Cable to continue to exist in the present after the future is supposed to be changed, there's only one explanation: he still comes back in time from the same point in the future.
Even though there's an assumption that Deadpool's sacrifice was enough to turn Russell off from killing The Headmaster and getting a real taste for murder, nothing in the future can possibly have changed for Cable to remain where he is. Russell still has to turn into the evil Firefist somewhere down the line and he still has to murder Cable's family in order for him to need to come back to the past.
That's the only way the ending of the movie genuinely works. Think about Back To The Future 2: in it, Old Biff travels back in time to 1955 to give his younger self the Sports Almanac and to completely change his future. When he returns to the future (which is impossible, but let's just allow it to fly for a moment), he doesn't simply continue existing in some splinter timeline. He literally vanishes (and painfully, for some reason).
The same should happen to Cable, as long as he genuinely changes the future. The fact that it doesn't all but confirms that he doesn't really change anything, whether the teddy bear changed or not.