Well, it's a cute aircraft indeed but it isn't popular on the 2nd hand market. Cyprus Turkish flied some 2nd hand ones, but got bored because it was difficult to maintain; while every airport has knowledge of 737 and A320. I don't think another major operator will pick them up. Prices of 2nd hand MD-90s will get lower... so no reason Boeing will continue to build new ones! Even China closes down its line because Chinese Airlines who can pick them up for lower cost aren't interested. So I guess Delte will stick to them for a while (because they have training and overhaul experience with it) and retire the 727, 737-200 and MD-80 first.
But you never know what happens to them in 10 years. Maybe FedEx will pick up all 120 built MD-90s... No-one would have thought they'd buy all Airbus 310-200s available; only 7 years ago not a single Airbus was flying cargo!
nobody has ever died from hard work, but why take the risk?