Las Vegas - Sheldon Adelson, the casino mogul, has seen his personal net worth plummet by more than USD 15 billion in recent months as Wall Street investors have grown more bearish about the prospects of casino companies that are pushing aggressively into Asia. That slide might put a crimp in his stated goal of surpassing Bill Gates and Warren Buffett, the only two titans ahead of him on Forbes' recent list of the richest people in the United States.