OP is 8 months away from having a kid, an 18-year financial commitment (or 22 if you pay for or at least subsidize college). Given current job satisfaction levels for this couple, they will not both be practicing law in Manhattan (or in any big-city firm) for another 18-22 years and may be near their income peak right now.
DC is a funny real estate market. Apart from the DC area being generally counter-cyclical, as someone pointed out, it is also subject to huge real estate turnover every time there is a new administration in the White House. The 2016 election will bring a new president along with the associated staff and trickle down; there will be thousands of cars plying local roads sporting license plates from the new president's home state. These people will buy homes. The ones who moved here for the current administration will probably stay, having cleverly burrowed their way into regular GS or ES jobs in the government and wanting to give some continuity to their kids who are in the local schools.
There are several problems with an old house in DC. DC gentrification has not spilled over into DC schools which range from marginal to downright scary. With a child on the way, even if the OP decides to move back to the DC they love, they will end up in NoVA or suburban MD where the schools are good. The only school advantage to DC is that your kids can pay in-state tuition at any state school in the country (as long as the family annual income is under a million dollars).
We've considered buying in DC to retire there, but have realized it makes no sense to own unless we occupy.
OP should assume getting rid of the old DC house is a given, but since it carries itself it might be wise to wait until early 2017 when there should be plenty of demand. If you have a good tenant, perhaps they would consider shifting to renting with an option to buy. Bear in mind that getting rid of a tenant who doesn't want to leave (not saying that's the case) can be very difficult in DC so there is always the possibility of taking a hit of several months rent while you go through that process.