It seems like such a logical plan, yet Lee rejected it and catastrophically lost. I know one reason Lee gave for rejecting it was that it might look like retreat, but wouldn't the Confederate victory on July 1 have made it not look like that?
I'm not trying to make this into a hypothetical question, but I guess I'm wondering why Longstreet wasn't given more credit at the time and still doesn't seem to be widely acknowledgement for that as well as his other advice at Gettysburg that was ignored but turned out to be completely correct, such as about Picket's Charge.
The idea of cutting off an army from its capital to force battle even seems supported by Sun Tzu's The Art of War:
If we wish to fight, the enemy can be forced to an engagement even though he be sheltered behind a high rampart and a deep ditch. All we need do is attack some other place that he will be obliged to relieve. (The Art of War 6:11)
There are several reasons why Longstreet's idea (I don't call it a plan, because it wasn't) to march down the Emmitsburg Road was unworkable, but I'll focus on the three big ones (they interconnect, but I'll deal with them in turn.
First, Lee's army was deployed on exterior lines, with Longstreet on the right, nearest to Washington, Hill in the center, and Ewell on the far left. While the road to Washington was near for Longstreet's corps, Ewell's would have to trek about one and a half miles to get on the Emmitsburg Road; Meade would be able to shift forces to his left much faster than Lee could get Ewell around to his right. Getting the army on the road would be a task in and of itself, not reckoning for the main Union army camped a stone's throw away.
Second, the army undertaking this maneuver would be vulnerable to a devastating flank attack; the US had no shortage of heavy artillery crowning the heights overlooking the road. Moreover, the largest corps in the Army of the Potomac, VI Corps under Sedgewick, was still not accounted for by Lee; his cavalry had run itself into the ground over the past week, and had not provided much useful intelligence, only arriving after the 2nd Day's fighting. If his army started down the Emmitsburg Road, it might crash into VI Corps, or if it was already on the battlefield, might be in position to overrun his column on the road. This column would be several miles long, and would take hours, or even days, to pass a single point in full view of the US formations along the heights. Armies in this period need to travel along multiple parallel roads to be in position to fight on a given day; piling three corps down a single road in full view of the enemy would be a recipe for disaster. Brooks Simpson has accused Lee of treating his enemy with contempt at Gettysburg, but Longstreet's plan is treating it like a map exercise.
Lastly, Lee has a wagon train stretching back to Cashtown, seventeen miles long, axles bending under the weight of goods requisitioned from the rich Pennsylvania countryside. Lee came north to destroy the Army of the Potomac, but his supply situation at his old base on the Rappahannock left him little choice; requisitioning as much food and fodder as possible and relieving Virginia of the pressure was a key secondary objective, and leaving his wagon train exposed could jeopardize the campaign as much as a lost battle.