UPS made this “philosophy” changes years ago. If a deferred product can be trucked in the same amount of time, it is much cheaper for the company. And the USPS is reducing its air footprint big time, in the name of saving money, DeJoy has been very clear about this. You will probably notice first class mail is taking longer. The much smaller air postal volume will be spread across many entities, also his desire to diversify.
This shift in delivery was bound to happen at some point. Painful for the Express side but it has always been the low hanging fruit that has yet to be picked. This combined with the “excess” pilots is not a fun situation.