As a guy who restored several airplanes and participated in a couple of early RVSM certifications, I can not imagine anyone trying to take a MD90 backwards to an earlier configuration. It would be a nightmare. While possibly legal, who knows if the parts even exist to dumb down an airplane to "State of the Art, Bicentennial Edition" ? Also, unless it was done under a prior approval, I doubt today's FAA Certification folks would sign off on an MD80 series type. The electrical systems don't tie together well and knowing Douglas, no one is entirely sure how the thing ever was designed to work in the first place. Those engineers are gone, scattered to the wind and too many work arounds were solved on the production floor (instead of by engineers that left a great paper trail). These sort of problems are why you see a bunch of airplanes parked in corrosion corners around the World.

One of the issues with the MD88's is that Delta's configuration is a little different. We were told to overhaul / repair some of the computers is astonishingly expensive. I would guess one reason aircraft get new flight decks is that the reliability, cost and availability is far better on stuff that is still in production.

There is too much good plug and use stuff available these days to believe Delta would spend millions trying to re-produce 1970's landfill fodder.

The 717 is a pretty radically different animal from an MD90.

End of the day, there are good reasons these airplanes ended up unloved orphans.