United airlines os one of the largest 737 operators in the world. Currently United does not have the equipment to replace the aging 737-291s. They are being D-Checked again, and should be here for a few more years.
The A320 joined United's fleet back in 1994 as a 727 replacement. It really did not replace that plane. This was before the 737NG was avalible. The A319 followed when orders for more airbusses were placed.
United will be operating 737s for a long time. I will not rule out the possiblity of more of them. Currently having 30 736s would do United well. There is no need for the 73Q, and the 738. Thats what the Airbusses are for.
United loves its 737s, and its one of their Largest Assests. If you make three flights on United, atleast one of them will be in a 737.
For the people who tell you that UA will never order anymore 737s, well, I'm not going to disagree with them. I do, however want to tell you that if it has to do with part support, or reducing the number of fleet types, that is pure bull shit. The 737s will be here for another 20 years, and UA will have a part stock for them, so adding more wont hurt them, or really create extra costs. The 737NGs are common to the 777. Never say never to more 737s, they may get a great deal from Boeing. You never know.