-90s is filling lost capacity systemwide from the DC-9 retirements but they are not necessarily replacing them. DC-9s were not doing the routes that the 90s are doing. The MD
-90s are pretty much "filling in" capacity as other a/c are shuffled around.
A more "direct" replacement that you are seeing for the 9s are large RJs and CRJs in some markets but again, they are not one-for-one replacements. As to the second part, no, they are not replacing 88s. The fleet has remained pretty much unchanged since the merger with some frames being retired/retured to lessors since bankruptcy. There were 128 pre-bk and now we're sitting at 117. Expect those to stay in the fleet till about 2018, is the number that's been thrown around. My best guess is that they will begin to exit around 2016 or so.
Just a couple of things to consider as an FYI:
had been selling 738s to 3rd parties upon delivery since around 2005 or 2006 and that stopped when they recieved their two newest 738s in 2010 (ships 3772 and 3773). It was a business decision because at the time they were hurting for cash and were not in a position to offer any meanigful domestic expansion.
-90s (in DL
conviguration) has the exact seating capacity as the 738 so essentially, they are getting the 737 capacity for rock bottom prices. I'm not going to pretend to know what they're paying for each fram but it's a fraction of what a factory 737 would cost. It's also nice because they are paying cash and do not have to worry about a very pricy twice-yearly mortagage payment on a new a/c. Sure, they can't fly JFK
but the majority of the 738 mission can be covered by the MD
-90 and that's on flights under 1600 miles.
[Edited 2012-01-05 18:07:18]
What gets measured gets done.