I potentially see PHX dropping to 50 or fewer flights within three years of the merger.
Maybe, maybe not. If part of the master plan is to evacuate the West Coast market then we'll likely see it. There's a lot more that goes into the viability of a hub than fares charged and proximity to other hubs, as important factors as those are they are but two variables in a complicated dynamic.
All hubs will see significant changes in routes, procedures, AC types and services offered as the two networks are merged into one. Redundancies will of course be eliminated, but frequencies to other routes may increase and new markets can be served from hubs that didn't make sense pre-merger.
It is true that only 390 miles separate LAX from PHX. What ends up happening in PHX depends to a large degree on to what kind of hub LAX were to become for the new AA and whether or not it can effectively absorb and execute the role and capacity PHX holds as a hub. For example if LAX were to be repurposed as a true Pacific gateway hub, with additional widebody routes and available capacity there going towards that end, would it also be able to function as a conduit for West Coast and Canadian travelers flying to Mexican destinations? Certainly, if it were a true hub a la DFW or ORD it would have that versatility, but with a market share of only 18.44% I think LAX would be hard-pressed to fulfill both roles. PHX has a robust international operation to/from Mexico while LAX flies only to SJD on their own metal; it seems a lot of their capacity as a "hub" in LAX depends on codeshare agreements.
For the period between July 2011 and June 2012 AA moved 8.4 million pax through LAX, while US moved 14.4 million pax through PHX. As far as Southwest is concerned, they moved 84.9% percent of the pax AA did through LAX and 83.8% of the pax US did through PHX in the same period, so the WN monkey on your back is in LAX as well. Between LAX and PHX, the operation in PHX can expand more easily and at a lower cost. It enjoys lower operating costs overall, less congestion and good weather.
Then there's DFW, some 900 miles to the East of PHX. With AA and Eagle it served some 42 million pax, about 3x what US did from PHX. Again, can DFW absorb the capacity and the role of PHX and is it cost-effective/profitable to do so? DFW's got it all as far as hubs are concerned, but is it an optimal use of its capacity (and aircraft) to connect folks from say SFO to PVR through there if LAX can't?
The new AA could pull out of PHX where it was the dominant carrier but then which competitors will fill the void and how will that affect the game in the West? There will be costs associated with staying in Phoenix, and costs associated with leaving. All of this depends however on the master network configuration Parker and friends are cooking up and the huge mess of calculations that go into determining hub profitability; plans and figures that none of us have access to, and even if I did I wouldn't presume to be able to make much sense of them. Anything's possible really. Maybe I'm due for a change of scenery anyways...
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Source on the numbers I used)