Why was the DC-9-80/MD-80 so successful despite being obsolete almost from birth? The McDonnell Douglas DC-9-80/MD-80 series of narrowbody jetliners was the second-to-last DC-9 major version produced (from 1979 through 1999), the second-largest, the most-produced (just under half of all DC-9s ever produced were of one of the five DC-9-80 minor versions), and the last to be powered by the Pratt &amp; Whitney JT8D low-bypass turbofan.

By 1985, however, the JT8D was already obsolete, with the first small-to-midsize-narrowbody-sized high-bypass turbofans having entered service in the first half of the 1980s, simultaneously offering greater fuel efficiency and lesser noise than the old, outdated JT8D.  Yet, despite this, the DC-9-80 only got more successful as the 1980s wore on and rolled over into the 1990s, with the single best year being 1991, and the years from 1987 through 1992 accounting for over half of all DC-9-80s produced.

Why and how was the DC-9-80 able to be so successful in the late 1980s and onwards, despite its 1960s-era engines rendering it obsolete just a few years after it entered service?
It's simple.  Cheap (development costs amortized decades before) and reliable.  They made money for airlines.  Or, you know, they wouldn't have bought them.  There's more to operating costs than fuel burn, and in any case, fuel prices in the mid 90s were cheaper than at any time since the 20s in constant dollars.  Fuel was so cheap that Air Canada was bringing L1011s out of mothballs in the mid 90s because they could operate them Toronto to LA profitably.