So there is an optimal amount of wealth to inherit to maximize human motivation, and it happens to be exactly the same amount that Gateses are going to give their children.

Who are you arguing against? I saw no one express the position that you're attacking.

would you simply reduce their inheritance to sane levels and more or less stop there?

Huh? Who stopped there? Do you have any reason to believe that the Gates handed their kids a "small" check and told them to get lost?