There's a more to a car trip than just going through a highway. Trips have endpoints. If you increase the size of your pipe into the city, you also have to increase the volume of the parking within the city. That means parking garages, lots of them. In addition to more costs for everybody, you're then deteriorating the urban fabric and making drivers within the city miserable in process (whether they recognize it or not).
As far as # of daily metro rides. My point is that if the metro were shutdown, the highways in and around DC would be a nightmare and people would not enjoy travelling in or through it. Getting around in DC, without the metro, would BE INTRACTABLE.
As for developers, while it is not "their job" to plan for infrastructure, it is very disturbing that they can simply buy land for cheap, put down and sell thousands of homes and walk away while forcing the nearby cities to deal with the mess they made. In response, many localities are now pushing "smart growth", we simply can't allow developers to make short-term selfish choices and pass the long-term costs onto everyone else.