Longstreet was correct, of course. The quality if Lee’s thinking as a commander deteriorated along with his decline in cardiovascular health during the war. Much of the post-war hostility against Longstreet was due to his embrace of reconciliation and Republican political loyalty.
Longstreet dragged his posterior on the second day and failed to take a wide-open Little Roundtop and potentially roll up the Union line. His tardiness allowed the Yankees to(barely)occupy LRT in time to repel the Rebel assaults.
There were a number of failures on both sides at Gettysburg.