If you did something as drastic to the timeline such as killing Hitler, your boomer parents would never have been born and you would not exist. In fact almost no one younger than a certain age that exists today would be around. The world would be full of different people.

An event like WW2 directly or indirectly affected everyone on the planet. Prevent it from happening and your grandparents, if they meet at all, copulate at different times and in different circumstances. Even a slight change in timing and circumstances of conception can lead to a different soul being formed.

It's hard to imagine Marty McFly's mom getting knocked up at the exact same time, 3 times in a row, with a much more confident daddy McFly strutting around.

I hate parties.