We will be seeing more of the Q400. CO is planning to use this to fly from EWR to DCA. The reason being the low attitude avoids the air congestion and delays.
But I think it will drive people to the train or the DL shuttle. The flights from DCA - EWR have less passengers than before and this will be the lynch pin. Even the 3pm DCA-EWR - popular for connectors to CO international desintations is going from a 735 to an RJ.
Turboprops from DCA - NYC are considered history. Years back I flew a DL prop plane from DCA to Bridgeport Conn - every week. But it bounced around so bad, I switch to flying CO from Bridgeport to EWR to DCA. At least the EWR-DCA was a jet and in those days a Y up fare was A class.