In a setting confirmed to have time travel, isn't it reasonable to assume the timeline already incorporates any efforts by time travellers to change the present? You can't change the past, because you are already part of it and time travel is generally a one off thing, you don't get a second go,* and even if you did that second go is also already part of the timeline.
*Unless you're Abaddon that one time he went to a daemon world where time flows backwards.