The way around this is to figure out how many total assets you need in each city and then reject any surplus from the AA end of things. Once you are merged, you are one carrier anyway and you combine what you have left in the same location. For example, suppose AA and US have a combined 20 gates in a particular city and it is determined you only need to keep 15. You reject 5 gates from the AA side and keep all the US gates. Once merged, you will move all your operations to one terminal anyway and the gate space will be combined as one.
I'm well aware of the concept. It's one thing to write the above in the abstract, but there are very few real world examples. In fact, I can't find one.
Could you list three cities where US + AA control a combined 20 gates that are not NYC, ORD, LAX, DFW, MIA, WAS, PHL, PHX or CLT?
And even if you could list three (or even one), as
eolesen pointed out, if the goal is for AA+US to grow and compete against UA and DL, then in which cities would the combined airline reduce the 20 gates to just 15?
If/when US and AA combine, the airline may have too many gates at PHX, CLT and PHL (yes, the AA gates would be surplus). And the combined airline would have surplus gates at LAX, DFW, ORD, MIA and JFK (all would be current US gates, and there's nothing that can be done with them short of a Ch 11 filing by US).
The combined airline wouldn't give up gates at LGA or DCA voluntarily, although government could force some asset divestitures.