There is another poster on this forum who seems to know some info regarding fleet plans. As of right now, I don't believe there is any plans to retire any Airbus narrowbody, including early build A320s. I believe Delta did receive a cycle extension for the oldest A320s. Yes, they did retire a few. However, Delta right now is focused on getting the entire M88 fleet out by the end of next year. The retirements can be divided into half. Half the remaining fleet leaves this year, the rest next year. C checks are done for the M88s. The M90 fleet is being parked temporarily as Delta is adding capacity in the 739/A321 deliveries. The M90s that have been parked are not just excess capacity being parked but cutting down engine maintenance. Only a shop in NZ is doing engine maintenance on the M90s. Delta will bring the M90 fleet down to about 20 before they come back temporarily in 2021 before the A321neos/A220-300s will finish them off by the end of 2023. Its also possible Delta could end up keeping them parked for good if maintenance/fuel becomes problematic.

For those who rip Delta for the 49 M90s they added. They actually paid for those aircraft on the cheap. Not lease, no relatively high cost order for 737-800s. Instead Delta was able to significantly reduce their debt load while park the old NW DC-9s. (The 717s were really an indirect 50 seat replacement rather than a DC-9 replacement. Now that Delta is in much better fiscal shape, they've been able to take relatively cheap, last off the line relatively speaking 737-900ER/A321ceos and park not just the oldest 757s and the domestic 767s but replace the M88/90s. By the end of 2023, the 737-900ER/A321ceo will be considered as primarily replacements for the M88/90 fleets rather than the 757/767s which Delta initially intended to replace when the initial order of 100 737-900ERs was announced. The A321neo is the true 757 replacement for Delta, especially on those longer routes. So in the end, Delta was able to get new technology and fuel efficiency of the A321neo to replace the 757 on those longer routes while the 737-900ER/A321ceos replace the M88/90 and the shorter 757 routes. (The domestic 767 replacement has essentially been the 757-300 while the remaining 757s have taken over routes the 757-300 initially was flying right after the merger.) If you go through previous Delta presentations, Delta has slides that explain how upgauging from old M88/90s to newer 737-900ER/A321ceos is a massive improvement on costs. The A320s fill in so to speak the gaps where the M88/90 once primarily flew and where 737-900ER/A321ceo is a bit too high in capacity. In regards to the A319, they are relatively new and its going to a while before Delta parks them. The A319 replacement will come on line at roughly the same time as the 737-800 replacement.

So the haters out there, just compare to what Delta has done with their narrowbody fleet to UA/AA. Delta is miles ahead of both in narrowbody fleet replacement. UA is catching up to what Delta has done especially with the 737-10 MAX order and how it will replace the oldest A320s. AA is hardly growing, especially with the massive debt load they incurred with the 2011 mega narrowbody order.