If that was the case there would be no real time-line, think about it, in the future everyone has the capability to go back in time to change whatever they want, this time line would simply stop existing as everyone would just go and change it to how they want it to be.
The temptation to make a "perfect timeline" would drive you mad, we'd break the fabric of space and time.
The whole point is to LEARN from the past, mistakes are made for a reason, things happen for a reason so you as a human can grow into something more, if you can simply go change everything you'll never learn or evolve.