Originally Posted by Zappity

At the outset of their last aircraft RFP for widebodies, the airline decided that they would not be considering any "Current Generation" aircraft as by the time they were delivered they would be "old technology". This includes the 777-200/300 and A330.

By 2025 the fleet will pretty much look like this:
A mix of 737NG's and 737MAX's (hundreds of aircraft) Most MAX's will be 9's, The first 737NG's to be retired will probably be the -700's and -800's as they are the oldest and have been flown the hardest. The -700's may be replaced by the largest C-series, but probably not until the C-series has ETOPS certification similar to the 737.
several dozen 787-8's and -9's (replacing 767's)
Mix of A350-900 and A350-1000's
A319/A320 fleet in the process of retirement and replacement by 737X
757-300 pretty much exclusively flying Hawaii routes
Some 777-200's and 767-400's may still be lingering around at that point, but they will be slated for retirement

United has a large and flexible fleet. They do not replace aircraft on a one-for-one basis. They instead build a fleet that has a range of capabilities and capacities for the routes they want to fly. They don't go out and say "we need to replace the 744's, let's find an aircraft with those same capabilities". They instead find the best aircraft for the missions that are most profitable for the company.