Lee did not destroy the Union army at Second Bull Run, or Fredericksburg, or during the Seven Days battles, or at Antietam or Chancellorsville. What makes you think he would have destroyed it in Pennsylvania?
At Chancellorsville, Lee confronted a Union Army of 130,000 with 60,000 men.
At Gettysburg, he had 70,000 men and the Union had about 95,000.
If he could reduce the Army of the Potomac's effectives by 30% after one battle with such great odds, he had good reason to believe he could have even more success on more even terms.
Lee's forces took almost 6,000 prisoners of war at Chancellorsville and had to let thousands of others go because they did not have the manpower.
Had he split Meade's army at Gettysburg, he could have done some absolutely brutal damage.