"catastrophic defeat of Meade's army would not have signaled an end to the war"

Given Lincoln's singlemindedness to prosecute the War to the fullest, even a Catastrophic Defeat wouldn't sway Lincoln.

The only thing that would have turned Lincoln's head on this matter would have been the Northern people turning out in force demanding an end of the war. Even then Lincoln did not believe he was going to be reelected in 1864. But that didn't stop Lincoln from pushing Grant to Richmond without regarding the high casuality that caused.