But if the United States does what it just -- exiling or executing every last Confederate officer or major official, and all the major planter sonsofbitches -- then suddenly the South is without an elite. Sure, the place would be dominated by Yankee capital, but a new political class would have to be created, and I doubt that, in the absence of the planters, this new class would resort to such extremes in preserving their own control over the South.
Why not? The planter class did lose power in the South toward the end of the century, but the new governments, elected by poorer whites, were if anything more racist than the planter ones.
Anyway you haven't explained why anyone needs to bother. With the war won, the job in hand was to reconcile the defeated South to the restored Union, which might otherwise be saddled with a gigantic "Irish problem". Pushing harder for Black rights would obstruct that process, not assist it. So the sensible thing was to let it drop and leave it to a later generation to worry about - which was exactly what happened.