yes there is. You can't accept that DL is not OBLIGATED to add flights but it can.
What will you say when DL does add additional flights besides ATL and DL becomes the 3rd carrier at DAL on a long-term basis?
and let's see what the court says but the DOT has made it clear that airport access laws are there to protect competition, not allow carriers to hold onto gates that they cannot or do not use. AA cannot serve DAL because of the DOJ merger agreement. By the time AA can add flights again, the chances are very high that DAL will be completely full.
and as to the question of why AA should serve DAL, they have the opportunity to serve MDW but do not. DL is pursuing the same strategy in DFW/DAL that they use from MDW/ORD where DL serves ATL, DTW, MSP, and SLC from both airports - and has the highest average fares and market share for the region to those cities.
AA has not pursued that strategy and neither has UA or WN, each of which operate hubs in both cities.