Well, yes. And of course that is what ended up happening.
But the point I'm making is this. It's correct that there is not a single case of a major field army being "destroyed" in a battle during the Civil War. No Cannae, to use the classic example. There are several reasons for this, but among them would be the fact that in every case of a severe defeat, the losing army had a line of retreat and space in which to retreat. So in the event of a defeat on the scale of, say, First or Second Manassas for the Union or Missionary Ridge or Nashville for the Confederacy, the losing army had the ability to run away to fight again another day.
In the Seven Days, however, that would have been quite difficult for the Army of the Potomac. The terrain was swampy, crisscrossed with rivers and creeks, with easy movement limited to a small number of roads. I've always had the feeling that, on the Peninsula and in front of Richmond, the divisions and corps of the Army of the Potomac are uncomfortably packed, like people on a crowded elevator.
So, hypothetically, if the Battle of Glendale had resulted in a Manassas-level defeat for the Union forces, it seems to me that Lee would have had the chance to inflict a decisive defeat that might have resulted in the effective destruction of the Army of the Potomac, because the Union forces would not have been able to easily run away. Perhaps Glendale, rather than Gettysburg, should be seen as Lee's great "missed opportunity" to win the war.