The DC-9 was always a pilot favorite. The rear mounted engines make the wing very efficient. The Boeing 717 which is based on the DC-9 is very efficient and reliable, but it lacks commonality with other Boeings, making it uneconomical for most airlines.
Delta is taking over all of AirTran’s 717’s after AirTran was acquired by Southwest. That’s probably why Delta is retiring the DC-9.
Airline mechanic here (A&P/FCC).., The DC-9 was a great a/c. The last of them Delta is retiring is the -50 version (lead sled).
The 717 is crap and very unreliable. Sure it sips fuel but leaves passengers and crew stranded.