My actual position is that:

1) Gateses had some true reason for donating most of the money -- probably a combination of "want to do a lot of good", "want to become famous", etc. -- and they decided that these goals are more important for them than maximizing the inheritance of their children. I am not criticizing them for making that decision; I think it is a correct one, or at least in a good direction.

2) But the explanation that they want their children to "make their own mark on the world" is most likely a rationalization of the previous paragraph. It's like, where the true version is "saving thousand human lives is more important for me than making my child twice as rich", this explanation is trying to add "...and coincidentally, not making my child twice as rich is actually better for my child, so actually I am optimizing for my child", which in my opinion is clearly false, but obviously socially preferable.

3) What specifically would one do to literally optimize for the chance that their children would "make their own mark on the world"? I am not going into details here, because that would depend on specific talents and interests of the child, but I believe it is a combination of giving them more resources; spending more resources on their teachers or coaches; spending my own time helping them with their own projects.

4) I can imagine being the child, and selfishly resenting that my parents did not optimize for me.

5) However I think that the child still has more money than necessary to have a great life.

My whole point is that (2) is a rationalization.