tells you more about the state of their finances than anything else I guess
Negative.
DL is in great financial shape currently and has consecutively posted the highest profits when compared to UA/US/AA.
The reason they are upgrading the MD88/90 is simple. They are all paid for. So although the fuel consumption is higher than a -700/800 they are not paying 300k-500k per month in lease costs (per aircraft).
That saves tons of cash, which in turn means more money for investments and gas...
Furthermore, they will soon be receiving the first of 100 -900ERs and already have 10 -700s, 73 -800s, 56 A319s, 69 A320s, and soon to be 88 717s.
Definitely a different approach to fleet planning.