Sean, same here.
I figure electricians and plumbers in their trades have conspired with local building code enforcers to make more money, and or more work for their members by improving the ground, and the chimeny flu liner diameter from 3, or 4 to 6 inches for gas water heaters. Expensive jobs. So if a 3 inch diameter flu was adequate for 80 years, and our grandparents never had asfixiation, or electrocution problems, why now? As usual, do the math, follow the money.