The primary reason for reduced chamber pressures and reduced payload, assuming a gun is safe to shoot full pressure loads, is to keep stress down on old wood. L.C. Smiths are renown for cracks behind the locks. Remington 1894&1900 are known for cracks in the head of the stock which lead to a split. The cost to repair or replace a stock may well exceed the total value of a double. Heavy loads can cause old stocks to crack or a gun to go off face. Barrel failures are dramatic events but thankfully a rare one as well.
