What I really worry about: Are the 737s capable of safely handling the flights to Hawaii under these circumstances? I realize that other airlines don't send planes with tons of excess fuel, but I can't help but thinking that a NW 753 would have flown this particular trip with a much greater margin. What would AS have done, for instance, if the headwinds had averaged 120 instead of 86 mph?
If they aren't going to make it, they'll divert. That's the entire point of ETOPS. As was pointed out upthread, a 120 MPH headwind becomes a 120 mile tailwind if things go south.
As for the 757 being 'over-engineered', I would point out that it was designed to most efficient and versatile on the higher end of the medium haul spectrum. Originally conceived as a kind 727-ER, it eventually morphed into a completely different type, essentially a 767 on a diet. The 737ER and 739ER come pretty close to filling the gap in the product line left by the discontinuation of the 757, however, I suspect whatever next generation narrow body type eventually replaces the 737 will look a lot like a slimmed down 787 with payload/range specs very similiar to the 757.
I'm saying that the 757 is over-engineered for replacing a 737NG, except for very few missions (high/hot, transcon where you end up with lots of headwinds)- more plane than you really want.
I suppose the 737 replacement might have a high end model that could match a 757 (kind of a version of the 739ER++)... but really, does WN need a 757 to go from ELP to DAL?