Existing service is 1x daily Alaska, 1x daily Delta on a Delta One 757 and 2x daily American A321. I've taken all of them and while Delta does serve this route as a Delta One product, the others are standard domestic routes. American offers slightly upgraded meals on the route in my experience but it's a standard domestic A321. Nearly every flight I've experienced has been rammed full. I think United could comfortably compete in the market. I'd take them over Alaska and American without hesitation.
I do as well, especially if it operated by a 737-800 or 737 MAX 8 but would rather have the 757 instead. I don't think UA could get another DCA-SFO frequency but certainly not DCA-DEN given it already is operated four times a day. More competition the better on DCA-LAX.