You post a good concern, but I don't think ORD or LAX will be scaled down by AA or UA, respectively.
At ORD, AA is experimenting with making its banks less crowded while running flights more or less continuously throughout the day, basically realigning its schedule there with no net loss in daily flights.
From an operational perspective, this means more productive use of labor, more automation, and generally less labor needed. As you can imagine, it leads to better operational performance on all counts.
From a marketing perspective, it is doing exactly what AA intended by acquiring the STL hub. Because connections through O'Hare are now on average twice as long as going through the typical bank-saturated hubs of DFW and STL, AA is trying to push connecting consumers to go through STL.
This all makes sense if AA is trying to capture more of ORD's O&D traffic, something which UA has had a lock on for years (it is the hometown carrier after all). It remains to be seen whether this move will be successful by AA, for it is alienating customers who are turned off by the long padded connections, increasing their journey time through ORD.
I've gotten to see this marketing tactic in action as I've selected to go through STL and DFW coming home for Thanksgiving from IND -- but AA doesn't want the low-fare purchaser like me going through the high yield hub it intends ORD to be; so from my perspective, it is working at making low-yield traffic connect through STL, and to a lesser extent, the flagship hub at DFW. But do they want to alienate ALL connecting traffic?
As for LAX by United, United has already shrunk there as much as it can without hurting its market share too much. Former shuttle operations have been turned into RJ operations... a great move. While it was sad to see LAX lose service to CDG (a no-brainer) and HKG (this has actually left quite a bit of market demand), it still has a very vital function in the United system.
While the LA import/export market has sagged, the high-roller O&D travelers are still there, making the hub still very lucrative. In addition to funneling these passengers onto important transcons to JFK, BOS and to international gateways at SFO, MIA, ORD, and IAD, there is still much market demand for service to the western cities either by mainline (SEA, PDX) or express (PHX, TUS). LAX will not be replaced as the Central American or South Pacific hub, either. Look for LAX-FRA service soon.
Express provides VERY strong feed from all over SoCal and much of the Southwest, and LAX is an important *alliance city.
Also, if UA were to cut any further service, AA's expanded and streamlined operations would start to eat away at UA's market share. Once the LAX expansion is in effect, expect UA to play a large part in the new terminal facilities.
no wire hangers!