From what I have heard and read, DL was not 100% satisfied with the performance of the MD-90; the MD-90s were being introduced just as the Boeing and McDonnell Douglas deal was coming together. Delta originally had a huge comittment to the MD-90, orders/options/commitments for over 100 aircraft and had planned to use it to replace the 727-200, 737-200,737-300 and, eventually, the MD-88. Due to the timing of the Boeing deal and DL being less than thrilled with its MD-90s, DL only took about 16 (?) of the MD-90s and then placed a major order for 737-800s and more 757-200s. The MD-90s, I beleive, will be around for a while, but look for them to leave the DL fleet after DL has completed is 72S fleet replacement program.