What prevents DC-9 series aircraft from being reengined with more-efficient engines? All DC-9s except for the MD-90\B717 series use old, inefficient Pratt &amp; Whitney JT8D series low-bypass turbofans, even the best of which have a paltry 1.74:1 bypass ratio.  We know that a DC-9 can accommodate a high-bypass turbofan; the final iteration of the DC-9 (the MD-90 series, including the Boeing 717) uses much more efficient engines with bypass ratios in the 4-5:1 range (which is the main thing distinguishing the 90-series aircraft from their predecessors).

Given that we know that a DC-9 is perfectly capable of using a high-bypass engine, and that the kerosene-guzzling nature of the JT8D is the main reason why DC-9s are being steadily phased out of service (as their low fuel efficiency makes it hard for airlines to make a profit from a pre-90 DC-9), why haven't the remaining older-series DC-9s been re-engined with high-bypass turbofans?
A re-engine program can be pretty expensive. This can make sense on military aircraft that take much longer to accumulate hours and cycles, or even cargo aircraft like the DC-8 in a similar position.

But the newest aircraft in the MD-80 series are going on 20 years at this point. They just don't have enough life left to justify that kind of expense. Most of them are not equipped with glass cockpits. When you add on the avionics upgrades needed, it becomes more cost effective to just buy a 737 or A320 which comes with the latest engines, avionics, and other features.

You're going to need a major manufacturer willing to take on the effort of an engine replacement program. That would point to Boeing or Airbus, and that would only cut into their existing narrow body sales.