Even the Redskins are having problems with attendance, the Super Bowl run hold over lasted 15 years, but it's gone, they need to win too fill FedEx.
The United not only had low ticket sales, but a very low percentage of actual tickets used, they don't release those numbers, but it's obvious when there aren't half the number of seats filled as the announced number. Historically this was not the case, if they sold 22,000 tickets there was hardly an open seat in the lower level.
Your point about losing fans in Baltimore is exactly why the city shouldn't pay for them to stay. They need the DC market more than the city needs them to spur development. If they want to move to a city where the average citizen shoots dope, let them go, MLS will expand back into DC sooner rather than later.