I'm not sure that a lack of historical three star generals being killed is an apples to apples comparison for determining if the game is gamey (Isn't the three star rank part of structuring the game more than recreating history?). After all, how many Union three star generals saw combat in the Civil War? My recollection is one, Grant. A Confederate three star general might command a corps as well as an army. Joe Johnston was seriously wounded and that allowed Lee to take command of the Army of Northern Virginia. Hooker had a near miss at Chancellorsville, Meade had to move his headquarters at Gettysburg because cannon fire was so heavy. Joe Johnston was standing very near Bishop Polk when he was killed near Marietta, Jackson and Longstreet were both shot leading corps larger than many armies, James McPherson was killed at Atlanta, Reynolds was in command of the field at Gettysburg, Sedgewick was not in command at Spotsylvania but certainly thought he was safe, Stuart was in command of the field at Yellow Tavern, Lee could very easily have been captured at the Wilderness when Federal troops appeared at his headquarters with no Confederate troops present. There were also a few instances of "Lee to the rear" late in the war to keep him safe. I suspect there are more I'm not thinking about.
Other senior generals being killed/wounded in American wars would include Benedict Arnold, Robert Ross, Edward Pakenham, Richard Montgomery.
Add to that the fact that you are making an assault of a type never actually made during the war (the DC fortifications would have been pretty stout and, even destroyed, fortifications tend to provide cover for troops) and its hard to know what the result should be. I do agree that it's at least very back luck, if not more, to lose two generals over a short period of time. I know I was unhappy when, in my game, Grant died in his first battle.