The metal bits aren't what go obsolete. The tooling to produce the engines, the frames, the aerodynamic surfaces were destroyed only after the planes were retired. 3D printing doesn't help build microchips, wiring boards, etc.
Could some of those parts have been produced better with 3D printing? Sure. Particularly inside the engines, there are very complex forms that are difficult to make subtractively. But the whole plane? Big simple forms are far stronger and consistent when stamped from rolled stock than sintered up from powder.
The reason those planes were retired is that new requirements emerged, and it was decided (rightly or wrongly) that a new design was the right way to meet them.