How would you know if we are on a timeline where intervention from the future has changed the past? In our timeline, it hasn't changed.
That is: The original timeline is timeline A. Travelers from the future come back, intervene, and the result is timeline B. But to those on timeline B, nothing has changed - it just looks the way timeline B always looked.
(Unless the intervention is blatant - people show up in a flying saucer and use death rays to assassinate Hitler before he becomes Chancellor, say...)