The problem is that the floorpans generally rust from the inside. Water leaks around the windshield seal or other areas and runs beneath the floor mats. As George has pointed out in the past, the material is acidic and when it gets wet the result is an acid solution resting on the floor. The acid eats through the metal over time. I suspect that anyone with good floor pans and the original floor covering would do well to pull up the covering and use a very good sealing material (such as POR or the equivalent) to protect the floor.