I'm not an airplane engineer, but I think a fair amount of it is technical debt, not the quality of the DC-9 design. For example, the 737 - a competing design that came out around the same time - is not only still being made, it's is getting prepped for another minor revision (the 737 Max), and the newest narrowbody airliner design in common worldwide usage (the A320 series) is going on 30 years old. The 717 was launched as the MD-95, a project at McDonnell Douglas to update the DC-9 series for the 90s, and Boeing (after merging with McDonnell Douglas) mostly just built out the already-sold MD order book and then shut the line down.
It would likely be very good for a manufacturer to make a new, clean-sheet design with updates like a carbon fiber fuselage, but the time needed to bring such a product to market would cause major problems for any company attempting such a project.
Several such craft exist at the bigger end of the regional jet market. There's the Sukhoi Superjet 100, for example: https://en.wikipedia.org/wiki/Sukhoi_Superjet_100 or the Embraer 190/195. These are both post-2000 airliners that have sold or been ordered in the hundreds (in the case of the Embraer E-jet family, total orders exceed a thousand), which carry around 100-110 passengers in 2+2 or 2+3 seating, and which are designed for much the same market niche.
The reason the 717 is popular is because the airframes are fully depreciated; the cost more to run than the newer jets, but are much cheaper to buy or lease. (Source: a guy I know who used to work in Finance for Allegiant, back before they began to diversity from MD-80 family planes.)
This is in addition to established offerings from Embraer and Bombardier (and some Airbus and Boeing at the very high end of the regional jet market with the A-318 and the smaller 737s). Each of these is more or less a clean-sheet design. The CSeries is already in flight testing.
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I'm not an airplane engineer, but I think a fair amount of it is technical debt, not the quality of the DC-9 design. For example, the 737 - a competing design that came out around the same time - is not only still being made, it's is getting prepped for another minor revision (the 737 Max), and the newest narrowbody airliner design in common worldwide usage (the A320 series) is going on 30 years old. The 717 was launched as the MD-95, a project at McDonnell Douglas to update the DC-9 series for the 90s, and Boeing (after merging with McDonnell Douglas) mostly just built out the already-sold MD order book and then shut the line down.
It would likely be very good for a manufacturer to make a new, clean-sheet design with updates like a carbon fiber fuselage, but the time needed to bring such a product to market would cause major problems for any company attempting such a project.
Several such craft exist at the bigger end of the regional jet market. There's the Sukhoi Superjet 100, for example: https://en.wikipedia.org/wiki/Sukhoi_Superjet_100 or the Embraer 190/195. These are both post-2000 airliners that have sold or been ordered in the hundreds (in the case of the Embraer E-jet family, total orders exceed a thousand), which carry around 100-110 passengers in 2+2 or 2+3 seating, and which are designed for much the same market niche.
The reason the 717 is popular is because the airframes are fully depreciated; the cost more to run than the newer jets, but are much cheaper to buy or lease. (Source: a guy I know who used to work in Finance for Allegiant, back before they began to diversity from MD-80 family planes.)
In fact, there is about to be a veritable boom in that particular market. In addition to the Sukhoi Superjet 100, you have:
* The Bombardier CSeries (https://en.wikipedia.org/wiki/Bombardier_CSeries)
* The Comac ARJ21 (https://en.wikipedia.org/wiki/Comac_ARJ21)
* The Irkut MC-21 (https://en.wikipedia.org/wiki/Irkut_MC-21)
* The Mitsubishi MRJ (https://en.wikipedia.org/wiki/Mitsubishi_Regional_Jet)
This is in addition to established offerings from Embraer and Bombardier (and some Airbus and Boeing at the very high end of the regional jet market with the A-318 and the smaller 737s). Each of these is more or less a clean-sheet design. The CSeries is already in flight testing.
Dornier was also trying to enter this market with a new design in the late 90s/early 2000s with the 728 series (https://en.wikipedia.org/wiki/Fairchild_Dornier_728_family), but they ran out of money.
The rear-engine design was pioneered in the 1950s by Sud Aviation with the SE 210 Caravelle. It was very successful for its time....
https://en.wikipedia.org/wiki/Sud_Aviation_Caravelle