A few things:

Just because those places are within narrowbody range doesn't mean a long-haul (e.g. transatlantic) service will work from DCA's runway. The 321 is a great plane, but it's not good on runway performance. It's not like the 757; and will require more runway to take off with a full load than DCA has.

And all of those example destinations are beyond the perimeter. Congress will not add more beyond-perimeter exemptions just so AA can operate international flights out of DCA. They want flights to and from their own districts. not Mexico City. AA already flies to the only Caribbean markets within the perimeter and with pre-clearance.

And furthermore, because DCA is not equipped for non-preclearance international fights, the airport authority will not add facilities to support those flights. The airport authority has already built those facilities - they are at Dulles. They are legally required by Congress to operate the two airports as a system, and a key element of that system is that the international flights go to Dulles.

So, Congress isn't going to do this. AA isn't going to push for it. The local governments don't want it, and the airports authority doesn't want it.