Even though the purchase of McDonnell Douglas killed off the MD-90, it never gained traction as a 727-200 replacement because fuel was cheap. American did not start receiving their 737-800 until 1999. Had the need arose sooner they likely would have gone with a large MD-90 order. Northwest and United both went with A320 series instead. Because of issues with MD-11 program, that also delayed MD-90 so by the time it actually went into service with Delta, Boeing 737-800 wasn’t far away