That's not true at all. People who are born into wealth very rarely lose it, and almost always multiply it. Because our whole system is designed to increase wealth, with wealth. Any lunkhead can make money with money, if he can get the excess money in the first place. And being born into great wealth pretty much guarantees that.
Add to that the fact that great wealth insulates people from reality to an amazing extent, and you can easily find wealthy adults with a fifth-graders level of maturity and worldliness that can maintain their level of wealth, and even increase it. As in fact, they don't have to know anything about finance, or even do anything at all. They can simply hire experts to do it all for them.
Donald Trump really is as stupid as he appears. And the fact that he's rich doesn't mitigate that at all. Other people take care of his money, for him. Just like they buy his groceries, cook his meals, clean his house, raise his kids, tell him where to go and when to be there and provide the vehicles to get him there. All he has to do is play "the Trump" for the cameras, and for the audience. He knows nothing of the lives of real people, and couldn't care less. He's nothing more than a professional TV personality. He even has to pay his wives to play the TV personality's wife, for him.
What does he really know about anything? Nothing. He doesn't need to. He's never needed to. He's always been a little rich boy play-acting the big rich man. He's the old cartoon character of 'Richie Rich' all grown up, and playing at politics, now. And he doesn't need to know anything more to play that role than he ever knew … which wasn't much.