I don’t understand the PMUA fleet being retired as soon as they were. Of the planes that were retired 36 or so were snapped up by Fed Ex. These jets had 25-30k cycles with 55-65k hours. Delta recently parked some 752’s with 45K cycles 95k. The 752 has a LOV OF 75K cycles/150k hrs according to the FAA. I do t understand that move. They are needed.
I see three things that went into the decisions back around the 2012-2013 time frame.
1) Fleet age. The vast majority of UA's 757s were built between 1989-1993. Most were were 20-24 years old at the time, and inside they were in need of a new interior. It was decision time. Do you spend the capital on increased maintenance and an interior refresh? Or do you cut the cord that would be cut soon anyway? UA chose the latter.
2) They still had value and found a buyer for many of them. The fact that FedEx was willing to pay a few million each was an incentive to sell them instead of initiating a refurbishment. Looking at it now, it looks like a good move in and of itself. There's dozens of same-age ex-AA 757s that didn't find another life.
3) The 757 was simply not a very economical plane for the domestic missions it was being used for and would have continued to be used on. This was even more true then when fuel was more expensive. The combination of higher fuel burn and rising maintenance costs leads to a high variable cost. In order to help mitigate the high costs, UA would likely have had to increase the seat density like DL did with an expensive interior refit. The economics simply weren't there to continue.
I think the loss of capacity during a crucial period of profits wasn't the most beneficial to UA, but this was a time when large capacity increases weren't being planned. A large 739 order was coming online, and something had to give. One can see why UA made the decision it did.