The reason I poked this thread was because I just finished Bruce Catton's "Mr Lincoln's Army" which pretty much agrees with your assessment.
Perhaps if Johnston hadn't been wounded at 7 pines and the command gone to Lee, or if McLellan had forged a sweep to Richmond as Kearny urged
I, Philip Kearny, an old soldier, enter my solemn protest against this order for retreat. We ought instead of retreating should follow up the enemy and take Richmond. And in full view of all responsible for such declaration, I say to you all, such an order can only be prompted by cowardice or treason.
Perhaps it would have been a short war.
Well, McClellan wasn't going to listen Kearny. McClellan had political influence, and since the Mexican War, had used his personal influence to his personal advantage. Had Kearny had impressive political influence (which he did not), he might have got a hearing.
McClellan created a wonderfully well-equipped and trained army, and he was thereafter loathe to risk it. He was also loathe to risk the reputation he had acquired. (Read again about the campaign in West Virginia--it was Rosecrans who kept things moving, it was Rosecrans who attacked when McClellan was actually preparing to withdraw--McClellan was timid and, as we say these days, "risk averse.")
If you look back, you'll see a passage in which i quoted Thomas Jackson telling a young officer to never take counsel of his fears. Jackson's career shows that this was bedrock in his military philosophy. McClellan constantly took counsel of his fears, he was horrified at the prospect of major defeat and failure. So when Allen Pinkerton told him there were 200,000 southern troops in Richmond, he was eager to believe.
The war winnowed out mediocrity, and both sides got what they wanted in the way of military men--eventually. It took years, though, and the so-called "Peter principle," that men are promoted past their competence, was demonstrated again and again. McClellan created a magnificent army--and he was paralyzed by the thought of what might happen to it (or more to the point, to himself) if he risked it and failed.
Different officers responded to these sorts of situations in different ways, and their responses can be very revealing. I'll try to take the time to lay that out.