When was the farthest west the MD-88s have been scheduled in the last five years? I know for a few weeks in 2017, maybe it was 2018, there was one on MSP-PDX.
I think they are quarantined to the eastern half of the country now. I don't know where the border of their domain is but I think it is a hard border for the fleet.
Part of the upgraded avionics the fleet was planning to get along with the ADS-B was an upgraded FMS system. This was a very involved upgrade to the entire electrical distribution system of the airframe. down in the lower e/e compartment there is a huge relay panel. It is the entire forward wall of the e/e. It's full. The new FMS needs more space on that panel. It is kind of like making an addition for you house, a home theater room would be a good example. Your electrical panel is maxed out for your current house. It also has had many years of crazy additions. Time for a gut and and expansion.
So what does this have to do with the eastern quarantine? The old FMS is way too small for today's database. Let's face it, it is a computer from the mid 80's that has been upgraded many times. It needs a larger hard drive essentially, but the motherboard can not accept anything larger. A newer FMS needs more power than the plane has available in it's current configuration. This was the true cause for retirement. Yes, ADS-B factors in. It wasn't economical to install the ungraded ADS-B system on a fleet that would be quarantined to half the country. A few airframes, 20 I believe, received ADS-B without the FMS upgrade. They are the last to go as their HMV due date is about a year after the ADS-B requirement.
That is the story I heard. I could be wrong,but It really makes sense. It explains why DL was buying up every spare part, engine, and entire parts airframes. At the time the plan was to upgrade the FMS and keep flying the MD80/90 for many years. The MD90 engine costs skyrocketed, then the deal for the FMS upgrade fell through.
With that I'm really going to piss some of you off again and say the 717 quickly follows the MD80/90 fleet out of the DL fleet. It's just my guess. I really see it as an orphan fleet that will go away. Yes, I'm well aware the BR engine is done in house. It doesn't bring in a ton of outside money. After 2020 I believe the bow wave of shop visits for this trails off. That space is very much needed for the PW GTF work which is one of the two future engines for DL.