Lately, Sam Jacobs has been having a lot of conversations with his family’s lawyers. He’s trying to gain access to more of his $30 million trust fund. At 25, he’s hit the age when many heirs can blow their money on harebrained businesses or a stable of sports cars. He doesn’t want to do that, but by wealth management standards, his plan is just as bad. He wants to give it all away. “I want to build a world where someone like me, a young person who controls tens of millions of dollars, is impossible,” he said. A socialist since...