In 1950, the year of the decennial census in which DC's population peaked, the District of Columbia was home to 47% of the region's residents. Over the next six decades, the region grew and the District's population decreased.
There's nothing particularly special about what has happened in the DC region. Suburbanization happened everywhere. So the following hypothetical is a kind of pointless "what if?" What if, from 1950-2010, DC proper's population had grown at the same rate as that of the metropolitan area?
From 1950-2010, the Washington region grew from 1,701,937 residents to 5,582,170, while the population of the District fell from 802,178 to 601,723. Now that the city is growing again, some are feeling cramped, feeling that the new residents are forcing out existing residents.
This notion that there is not enough room for everyone suggests that we are somehow approaching maximum capacity for the city, which is not completely untrue, given existing land use regulations. Under current zoning, and factoring the development of unused sites, maybe the population capacity of DC is 800,000; maybe 1 million? But given that restriction, the region will continue to grow horizontally and the District will continue to become home to a smaller share of the metropolitan population.
Suppose none of the factors that caused cities to suburbanize had ever occurred. (I realize that is equivalent to asking you to ignore a half century of American history, but just play along.) If, from 1950 to 2010, DC had maintained its share of the regional population, we would now have approximately 2.6 million people living in our city, making the city more than 4 times as dense as we are today.
If we are outraged by even the thought of easing the city's height restrictions a few stories, then this scenario must sound apocalyptic. And DC would certainly have a different character. But it is interesting to imagine what sort of benefits it potentially could have on the environment and the affordability of housing, two increasingly distressful dilemmas we face today.