Fuselage common rust area, why?

[Rustyrodknocker]

I saved this panel from my 73 Newport parts car since it is in good shape. It was unpainted on the under side. My 69 300 was absolutley shot and I rebuilt the whole bottom lip.
A thing to consider, is that these cars in their current shape, are the result of 50 years of aging.
Today we can do a much better job with better materials and take better care of them.
Given that,

they could hold up for decades.

 

[Chrome58]

I'm in the exact same spot right now with my 70 Sport Fury... Before working on the trunk, I decided fixing the rear window was a priority. Someone very stupid did a butcher of a job (and I'm trying to stay polite) trying to hide and conceal the rusted area. I had to remove all that junk and now I can start afresh.