Your usually good info is off on this - there is a large spread in the number of cycles remaining on the DC9 fleet - true that some will time out in the next few years (and most have already been retired), and equally true that half the remaining (of the original ~173) will be good until ~2020 if navaid updates are made.

The real question is whether 1) navaid updates will actually be required, 2) whether DAL chooses to perform them or not, 3) the price of oil, 4) the state of economy and load factors, and 5) the availability of other next generation 100 seat lift.

Anyone who supposes to have the definitive answer to these and the immediate future of the DC9 based on 10 yr old articles, arbitration testimony, and CP scuttlebutt is deluding themselves.

I'll venture the fleet planners don't even no which direction they are going yet.