The problem is a bit more compound than that. Old isn't a problem. Well… Spare for airframe fatigue. Eventually, you end up having to rebuild the airframe due to stress fractures in the alloys…. But notwithstanding…
The problem is that the manufacturing capability has virtually vanished. It isn't that the planes are old. It is that no one is making the replacement parts. Why weren't they?
In today's era of CAD-CAM and breakthroughs in manufacturing technology… Why is it that new parts are so hard to make… Or new designs so hard to retrofit into the airframes?
The answer is one of willingness, not capability.