The thing is, suppose you went back in time and gave Bill Gates some startup capital. You think, I'll get rich from the stock. So now he has the money he wanted to use to start a different kind of company and there is no MS.
Not sure how to play it safe if you can travel in time. You never really know what will happen.
I am reminded of an episode of Night Court. Something about time travel always remained in my mind. The short version is Dan Fielding sees himself lying in the lobby. It is raining out side and so Dan decideds to escape using the fire escape.
Someone was after him, as I recall. Wanted him dead as I recall.
One of the other cast members poses this question: Did you die because you were shot, or did you die because you fell off the fire escape and they dragged your body inside?