Water from the cowl drains to either side around the firewall between the fire wall and the fender. there are two holes drilled in the bottom edge of the fender that drains the water: up until those holes get clogged with leaves and dirt.
It is almost as if the factory engineers wanted these cars off the road because they rusted out; creating another for them to sell a new car to. They also changed the trim and appearance of the car annually to motivate those with a good job to trade in their old car every two to three years.
Are the holes in the cowl the same place the bottom of the fender bolts up? It appears as though there is a nipple there. On one side of my car, there was a toggle like fixture up in the hole, and on the other side. . . . . well, there was what looks like a tiny wrench with bolts welded to it, with one going up in the frame, and the other going into a different hole in the fender, tacked to the frame. The bottom of the cowl is rotted out on both sides, and only the passenger's side has part of the 'nipple' left. I looked at one in a yard, but it's just one big rusted mess. I don't know if it is supposed to drain out around the mounting/adjusting bolt or not. It doesn't seem as if it could get out the front part, and there is a hole in the rear part that would suggest that it drains out there - but would seem incredibly dumb.
I'm very familiar with 'planned obsolescence' and they don't tell you in comes in two stages. The most obvious part is appearance, they want you to think your car is outdated about the time you could possibly afford a new one. The second, and unadvertised part, is they use the cheapest materials to wear out faster. Added to this, in the case of my 55 - they made a million and a half vehicles in 10 plants in 12 months. There were most likely better ways to make things, but not faster. As example, I offer the roofline and drip rails. When you drive in the rain, water gets in the 'seam' between the drip rail and the inner panel where the roof sweeps down. Everything looks fine until you take out the headliner and rear window to see what 60 years worth of rust looks like. After we fixed all that, we sealed up that 'seam' smooth with putty. You wouldn't know it if you didn't look for it.
As for your reference - fenders rusted out behind the front wheels for a couple decades before they addressed it. I tell people that the person who bought my 55 new was supposed to buy a 57 or 58 new, and the 55 would be sold a couple times as a used car, and by 65 be in a junk yard somewhere - and for all I know, it did. It isn't there now, and won't be for a longer while. I'm guessing the Impala sat for a stretch sometime, as when I took off the driver's side fender, there was the familiar 'nest' around where the body mounts attach at the cowl.
As for planned obsolescence, the gov't ruined that by mandating that cars last 100K miles. Between that and 'international platforms' they all look the same, can't tell a make or year on anything anymore.