I somewhat wonder why AA even bothers with IAD at this point. It's sad-AA (well Piedmont) has some incredibly friendly and caring customer service agents there.
WN adding IAD-DAL nonstop service might be a possibility with
(a) DFW having significant demand from IAD prior to the COVID-19 pandemic,
(b) WN having a significant FF base in both the WAS and DFW/DAL markets to support IAD-DAL nonstop service on WN, and
(c) there is a significant amount of population on the west side of the Washington Metropolitan Area (including in Loudoun County, Prince William County, Manassas, Manassas Park, and western Fairfax County).
One reason why AA still maintains a presence at IAD is due to the significant amount of population on the west side of the Washington Metropolitan Area. AA also had over 50% market share on O&D on both IAD-CLT and IAD-DFW through at least Q4 2020.
AA's oneworld partners BA, IB, QR, and AT serve IAD, but there is the AA FF base in DC, the BA/IB FF base in Europe, the QR FF base in the Middle East, and the AT FF base in Africa to support BA/IB/QR/AT service out of IAD.
While AA doesn't serve ORD, LAX, MIA, LGA, JFK, PHL, or PHX nonstop from IAD, AA still serves its ORD, LAX, MIA, LGA, JFK, PHL, and PHX hubs nonstop from its DCA hub.
There is less of a need for AA to serve PHL nonstop from IAD with
(a) the majority of the traffic on AA PHL-DCA nonstop flights and UA PHL-IAD nonstop flights being connecting traffic,
(b) AA already serving most of its Northeastern destinations nonstop from DCA, and
(c) AA's partners BA and IB being able to connect passengers to other European destinations from IAD through European hubs.
AA is also carrying some connecting traffic to the Northeastern U.S., Eastern Canada, and Europe from DCA through PHL along with some connecting traffic to the Southeastern U.S. from PHL through DCA.
AA could re-add IAD-ORD/MIA nonstop service and add IAD-PHX nonstop service once demand recovers in order to be more competitive in the WAS market, and AA has a FF base in the WAS market (due to the AA DCA hub and former US BWI hub) to support IAD-ORD/MIA/PHX nonstop service on AA.
HP and US both previously served PHX nonstop from IAD, but US dropped IAD-PHX nonstop service prior to the AA-US merger and back when US and UA were both in Star Alliance. The situation has changed since US dropped IAD-PHX nonstop service with the AA-US merger, the discontinuation of the US-UA partnership, the post-merger AA remaining in the oneworld alliance, and the significant FF bases that AA gained in both markets as a result of the AA-US merger.
WN adding IAD-PHX nonstop service might be a possibility with (a) the FF base that WN has in both PHX and WAS markets, (b) PHX being outside of the DCA perimeter, (c) IAD being far enough from BWI with IAD being almost 60 miles from BWI, and (d) connecting opportunities available to LAS and Southern California through PHX on WN.