"I'd assumed in aeronautics 60 years old meant well understood"
Sure, but it doesn't mean a design could become outdated anyway. The 737 design has the wing/ground clearance issue, which wasn't envisioned when jet engines had far smaller diameters.
Changing it's not simple, because moving it will require to re-design the wing box which is one of the most critical structural designs in a plane, aerodynamics will change, probably impacting the wing and tail surfaces position and design, actuators will need to change as well, etc etc.
Then parts and assembly lines needs to be updated (and workers as well) - plus the certification.
It's a lot of money - exactly the reason why Boeing attempted a shortcut, to increase profits.