DC was never meant to be a state. That's by design. It's further evidence of the power meant to be held by individual states as it demonstrates why the FFs didn't keep Philadelphia or New York or one of several other cities as the nation's capital. They didn't want the appearance of favoritism. It's a district, and it should remain a district. It would make more sense to allow any permanent occupants of DC to apply to either Maryland or Virginia for citizenship, thus allowing them representation. Besides, I believe the existence of DC is provided for by the Constitution, meaning we'd need an amendment for it to become a state. There's no need for that to happen.
I think there's a better argument for making U.S. territories states, IMO.