I've been thinking about this off-and-on for a while: the changes to the time stream propagate no faster than any effect in time usually does. Go back forty years or so, kill John/Sarah/whatever, and then return to the future from whence you came; nothing will have changed. However, wait it out the whole forty years, and the future you're living in will be different from the one you came from. No matter whether the original Terminators attack from forty years ahead, eighty years ahead etc, nothing will change for them.

This isn't alternate time tracks or the Men Who Murdered Mohamed but something in between. Not having seen the series, I don't know whether this version of time travel would fit the facts.