Ok...lets get some things straight...NOW.
First about 75 737s are dedicated to Shuttle operation. These planes have been outfitted with special galleys that allow the plane to fly for half of the day with out being restocked. There is also a refridgerator, and a trash compactor on all of teh Shuttle 737s.
The oldest 733 is only about 13 years old. It still has a lot of years left. There is no plans on retiring these birds anytime soon, as they do their job cheaply, efficintly, and very well.
Next, the A319 will not be joing UA shuttle anytime soon as the pilots union will not allow it. Why? Good question...different pay scales between 737 and A319.
Regarding the 757... while the only route that would require a plane of this size is SFO-LAX, lets be realisitc. It wont happen anytime soon. We need them on mainline, not on shuttle. UA operates aporx. 3 flights from LAX to SFo per hour. Some of those flights have 40 people on them, some have 126. It all depends. When they are full, they are making money (at least for the most part), when they are empty, they are not. A good plane is a good thing for United.
The 757 could do 25 minute turns. Thats easy. Simply do not refeul it after each flight, and have enough beverages stokced in the galleys for 5 flights. Just like on the Shuttle 737s. Also, multiple boarding can be done. Hey, if you wanted to, you could use all three doors.
United Shuttle serves the following destinations:
SFO, LAX, ONT, SAN, SBA, BUR, PSP, LAS, PHX, TUC, RNO, SMF, MFR, EUG, PDX, BOI, SEA, GEG, and SLC. As I said before, SFO to LAX is the only one that warrents a 757. The others like SFO-MFR, actually have UA Express serving most of the flights.
So, UA wont be operating 757s on Shuttle. But, what about putting some mainline flights in during peak hours? Good idea! At 8am, I would not mind seeing a 757, and a 737 go to LAX to relieve some of the congestion in SFO. You could dump the 8:30, and mpve half of the 8:45 flight passengers on to that 757. It would be full. Two flights would be cancled. Very good! The other half of people from the 8:45 flight would go at 9:00am.
Regarding the 737NG. I have been saying for a long time, to never rule out the possibility. I have my mixed thoughts on this. Right now, if Boeing approached UA with the 736, I know that they would look at it. There is really no need for any of the others right now.
Also, the 737-291s...ahh these little bundles of joy. Guess what...they are going thorugh Heavy Maintence again and should be here for another 3-5 years. Why...there is no replacement for these yet. None.
BTW...this is a little heads up for y'all. The A320 is not a 737 replacement for United, it never was and probably never will be. It was here to replace the 727s. That is what they are doing. All of what they are doing. The A319 was and still is United's plane of growth. It has opened up new markets for us such as SFO-BDL, and LAX-PIT which may have not been economical with a 757 before. The A319 is not a 737 replacement.