Ironically the 737-900ER CASM are not significantly lower than a 757-200. According to
http://www.aircraft-commerce.com/sam...g_2_sample.pdf the CASM of the 757-200 is about 7.85 and the 737-900ER 7.31 cents.
Considering the route flexibility of the 757 when compared to the 737 one wonders why many of the UAL managers think it's the best thing since sliced bread.
The only real answer, and the leaders of the industry seem to agree, is there really wasn't a replacement for the aging 757-200 fleet and Boeing only offered the -900ER in the near term.
What I find most interesting is on a CASM basis (according to the PDF) the A321-200 is the airplane to go with. But, when your management has made a deal with Boeing to keep it (the airline) in the air a bit longer Airbus wasn't considered. Of course this Kirby guy and the fact the Boeing deals (with Jeff S) seem to be history maybe there will be an order for some Airbuses in United's future.
However, for us working pilots remember the mantra, "bid what you are willing to fly and fly what you bid!"
***Please note***
Seating in the back of a full 757-200 to Hawaii is no different than doing the same in a 737. It's just more likely standbys will get on the 757.