The U.S. attorney is one of the most consequential figures in the District of Columbia’s criminal justice system. Washington might soon learn that lesson firsthand after Donald Trump’s eventual choice for the role — that is, his handpicked sheriff — assumes the reins from U.S. Attorney Matthew M. Graves. This week, Graves announced that he will officially step down on Jan. 16, four days before Trump steps up to take the oath of office.A cautious and understated prosecutor, Graves will leave with numerous accomplishments under his belt. His targeting of repeat offenders has paid off with reductions in gun- and...