49 MD88s were already scheduled to all be retired by the end of 2020. They were only really sticking around through 2019 to fly for peak summer 2020, which prior to this pandemic was predicted to sent record traffic numbers.
30 MD90s were really only going to stick around to help cover peak summer 2021 flying and help bridge the wind-down of the MD88/90 fleet
18 77E/77L too many large widebodies for the next several years, also in light of the A350s on order. It would be costly and very sub-optimal to run with very small fleets of both A350, 77E, 77L and literally put brand new A350s in storage. 777 retirement takes out 2 subfleets, 2 engine types, and eliminates a pilot group.category.
10 73Gs I am guessing this is more a function of timing if heavy maintenance was coming due since they are all in the 10-12 year range. As others said, give the impression they are doing something. Removing subfleets always makes investors happy. Can probably actually sell these aircraft versus older A320s which will be part-outs