Markets evolve. Some markets that were previously served by smaller equipment saw rising demand that required larger aircraft to meet that demand. Some went the other way, and some proved to no longer be financially viable and were dropped, allowing assets to be shifted to other places. Global air traffic was growing rapidly before COVID and most analysts think that trend will continue after we recover from the pandemic. Major hubs are saturated on a good day and go into meltdown mode when there is weather. Due to this, only way to meet rising demand is with aircraft that have larger seating capacity. United has chosen what they think will be the best path forward. They know that the single class 50 seaters are very unpopular, they are inefficient, as they age the cost of maintaining mechanical reliability significantly increases, and there is no real replacement other than the 550 and dealing with their performance limitations. They also know that United pilots are not going to relax scope and since they have shown that they don’t consider a new SNB to be the best option, there will be no increase in 70/76 seat aircraft. Given the decisions made by management, UAX will shrink as the single class 50 seaters are retired, and UAL will then take back that flying.

Much of the big aircraft order is to accomplish the same things with the mainline fleet. Bigger 737’s will replace smaller ones on certain routes, those smaller jets will then allow the addition of seats to other markets, older and less efficient aircraft such as the 757 are becoming more expensive to maintain will be replaced. United is updating their entire fleet, not focusing on UAL vs UAX.