The only "dumb" question is the one that is not asked.
rjemery,
Your question is not dumb at all; in fact it is a good one. As it has already been pointed out the DC-9 design is forty years old. Therefore it is unable to compete with new engineering, technology and composites. This is especially true in the wing design and the engines.
As we age it gets more expensive to keep us running, so it is with aircraft. At some point it becomes cost prohibitive to maintain an airframe any longer. Our company recently retired their fleet of 737-200 jets for that reason.
It would be possible to put new generation engines on the DC-9 airframe. However the airline would need to pay for the certification process with the governmental regulatory authorities. When it was all done you would have a several thousand-dollar saddle on a hundred dollar pony (substitute your monitory currency of choice). You still have the old wing with its poorer performance, older and heavier airframe with its associated higher maintenance needs. So it turns out that you were right all along the jets had served out their useful lives and needed to be retired.