[excuse the layman question] what exactly is the benefit to the military helicopter training in airspace that is heavily trafficked by commercial flights? What are the downsides to using empty airspace with simulated traffic for most training? Is it mainly that they don't get to learn the visual cues from being above the actual Potomac?
They're training in the same terrain where they'd be required to operate if their mission was to evacuate Washington VIPs in the event of an emergency.
in an actual emergency, you would divert traffic from DCA – the logic here is absurd
You don't believe the military helicopters involved in this hypothetical evacuation would fly in and out of DCA, do you? They're flying out of Andrews (and other military bases, I assume).
This isn't to say that the situation at Reagan wasn't stupid and a disaster waiting to happen but military pilots need to be reasonably be expected to operate in far more chaotic and busy airspace if the need arises so it makes sense that the .mil thinks nothing of routinely tossing them into very busy airspace.
That is like using civilians as NPC.
How are the fancy people in Washington going to get around if they can't take a helicopter Uber?