There is an implication in Longstreet's remarks that if McClellan had pushed his advantages at Antietam Creek, perhaps the war would have been shortened and lives saved.
But which way? It is possible that McClellan's army could have broken.
Criticism rests with the idea that McClellan had a large unengaged force (called, erroneously, by some observers a reserve), and if he'd only have attacked with that Lee's lines would be broken. There are a bunch of problems with this.
1. McClellan's unengaged force was smaller than people think, and Lee had more fresh troops than people think
McClellan's unengaged force was, in round figures, 11,000 infantry and 2 batteries. Two brigades counted as "engaged" would have been available, but Warren's little half-brigade wouldn't. 5th and 6th Corps could have launched roughly 14,000 infantry, or about the same strength as 2nd Corps on its own launched earlier in the day.
6th Corps would have been attacking two fairly fresh divisions head on in defensive terrain; Walker's and McLaws' divisions only action had been overrunning Sedgwick in the West Woods, and they'd had hours to reorganise after their victory. In addition Armistead's fresh brigade and the residue of Jackson's and Hood's divisions, and Early's brigade, were available.
For 6th Corps the question becomes "can 7,500 bayonets successfully frontally attack ca. 10,000 fresh bayonets back by maybe 5,000 used troops in defensive terrain)".
5th Corps with 6,500 bayonets would basically be attacking the Hagerstown Road position DH Hill rallied on when the sunken road collapsed, with 4 of RH Anderson's fresh brigades (ca. 4,000 bayonets), plus the residue of 3 of DH Hill's brigades holding a stone fence. To reach the stone fence they'd need to march a mile under the fire of ca. 10 guns, and in the last quarter mile (400 m) some 50-60 guns could fire on them. Unlike the Dunker Church position 6th Corps would be attacking, this position was well protected by artillery.
I'd personally contend that Franklin's two divisions (6 fresh bdes) had no chance of dislodging a superior enemy force (9 fresh brigades) in good defensive terrain. I don't think that had Porter's 5 bdes attacked the Hagerstown Road position (manned by 5 fresh and 3 used bdes) they'd have had much success either.
2. When to use the last reserve
A general should never throw in a last reserve into an attack. The last reserve is only to be used if the enemy force has broken, for a pursuit. If you throw in your last reserve to "break the enemy line" then you've nothing to push through the gap and any victory is hollow. A classic example would be Nashville, where Thomas "put everything in" and had nothing left to exploit when the Shy's Hill position was overrun, and Hood's army was able to make a clean break.
3. What if...
Assume McClellan did make one last roll of the dice, and 5th and 6th Corps were both bloodily repulsed (as you'd expect from the situation). Then what?