It was a good war tactic before the use of accurate firearms. People do not just do things for no reason at all. There is a rational reason for why things happen – especially when looking at systems. Do you know how pointless trench warfare would be with muskets? Nobody would ever die. Military tactics change. Economic factors change. People change with the times.
However, the "walking line" tactic was still used a surprising amount of time after it had become obsolete. It took some time for armies to realize that modern technology had made that tactic pointless.
Also, what if you walked in a straight-line towards sheltered enemies (say, behind a short wall, or in a trench)? They'd be able to fire a volley, then duck and cover. While you might not take insane losses, you'd take more than they did before you even got to the actual fight (which would probably be melee). Of course, guns used to not a long enough range to make the losses severe; but you got to admit... it seems like a bad idea.
This somewhat fleshes out the analogy, even. While the tactic used to not be all that bad, now technology had made it relatively inefficient.
Industry doesn't just spring up by itself. If we hadn't had a successful agricultural sector, then we would have been wasting time and capital getting those products from elsewhere. Our industry was also dependent on the resources had by slave labor. Textiles don't magically appear from factories. Slavery had a hand in putting us where we are today, just like every other successful turn of our nation. We build from what we have.
I figured you would come back with this, because it is a valid point. We do build on what we have.
But there is so much America has to build on in the past that would fuel it to become a superpower beyond slavery. Slavery happened, and slavery probably helped, but I wouldn't consider it a factor as big as WWII or our dreams of manifest destiny (and our resulting conquests towards the Pacific).
I suppose you could even argue slavery hurt us more than helped, as without it, we wouldn't have had the Civil War; but that is getting far too technical. Who knows, perhaps the large plantations would've been smaller and still been an effective source of agriculture due to the poor whites that couldn't compete with slaves nor buy slaves or land of their own actually having their own soil to till. But then again, maybe that would have slowed our western expansion, hurting us. Now I think I've ventured into pure speculation, but anyway...
Besides, why say slavery when you could expand it to include the "wage slavery" as well. Overall oppression of workers, so to speak. Then you'll have what fuelled both our agricultural and industrial sectors for quite a long time, beyond the Civil War.
I might've rambled a bit much, but I think it's readable.