IAD international pax is like wow! It ranks in the top 10 international gateways in the US.
That's always been the case at IAD. There's a greater diversity in carriers at IAD now than there used to be, but UA int'l capacity is down from a decade ago. Not massively so, but there used to be a few daily 744s and more 772s.
DCA is another story - with such limited gates, perimeter rules and a single runway, the airport functions very well I must say.
From my perspective, DCA copes as well as it can, which is about as polite as I can be! DCA is almost a constant headache from a traffic management perspective. Any amount of weather and we're in holding. A VIP movement at ADW and we're in holding. A flyover at Arlington and we're in holding. The airfield starts to saturate and we're in holding. They de-ice and we're in holding... You get the point. We go into holding almost daily for much of the year. The fact that it is effectively a single runway op with limited ramp space means that things are run as well as possible, but we're talking miles and miles from being efficient.
AA has done a good job in identifying good routes
AA definitely does not get to pick the routes! Every major city pair has an ATC pref route. A good example of an airline filing a totally wrong route every single day is the CommutAir flight from FAY-IAD. They file FAY.V56.WALLO..TYI..THHMP.CAVLR3. Under no circumstances would they be allowed to fly the THHMP.CAVLR3 transition, so every day it has to be changed to DORRN.CAVLR3, which is the pref route. It doesn't seem like a big difference looking at it on a map, but flying over THHMP means is a big problem for DCA arrivals. THHMP is still a published transition, but it's almost exclusively used for ORF departures because it doesn't conflict with DCA arrivals.
So, airlines can file whatever routes they'd like, but they're not gonna get it if it's a bad route for ATC. It's rare to see an airline get too creative with routes unless you're talking about longer distances...they pretty much always go with the pref routes. Pilots don't want to have to constantly be putting new routes in the FMS, and it's draining from an ATC perspective to have to constantly be reading reroutes and tying up the frequency.