1) Powell suggests that slavery might have ended on its own because the antebellum South was approaching economic collapse. Nothing could be further than the truth. In just one comparison from the 1860 census, railroad miles in Southern states had increased four-fold, making it easier and cheaper to transport cotton to market. Slavery might have ended by a means other than war, but in 1860 Southern commercial activity was increasing, not failing.
2) Lincoln's war aims were political as much as military. Whigs like Lincoln and his political model, Henry Clay, had been frustrated by Southern Democrats since the presidency of Andrew Jackson. Destruction of Southern political and economic power allowed the Republican Party (successor to the Whigs) to reshape the United States in its own image. Chapter seven of Michael Lind's book, What Lincoln Believed: The Values and Convictions of America's Greatest President provides specifics on the passage of high tariffs, a national banking system and subsidies for railroads and industries.
In other words, the Civil War had far less to do with civil rights for emancipated slaves than regional control of the United States government. For example, Lind points out that in every presidential election from 1868 until 1960, at least one of the two presidential candidates from the two major parties came from one of five states: Ohio, New York, Illinois, Indiana and Massachusetts. My own study of states represented by presidential and vice presidential candidates shows that sixty-three of ninety-six candidates 1868 until 1960 came from Ohio, New York, Illinois, Indiana and Massachusetts. Men from the South or the West during those ninety-two years rarely exercised national political leadership.
3) War costs were not evenly shared. If I may quote Lind (page 240),
The South had been ruined by the Civil War. Two hundred fifty thousand Confederates died of wounds from battle or disease. In the Union forces had suffered proportional losses, instead of losing 360,000 men, they would have lost more than a million. The equivalent to the South's losses by the United States in World War II, in which the nation lost more than 300,000, would have been more than six million. In addition to conquest and occupation, Southerners experienced losses on a scale suffered by European nation in the world wars of the twentieth century. The abolition of slavery erased $2 billion of capital. Two-thirds of Southern wealth vanished. Between 1860 and 1870 the South's share of U.S. wealth shrank from 30 percent to 12 percent. By 1880 the gap in per capita wealth between the North and South was comparable to that between Germany and Russia.
On the next page Lind writes, "Poor black and white Southerners suffered the most from the conversion of the conquered South into an unfairly taxed resource colony of the booming industrial Northeast under the Republicans who succeeded Lincoln in the White House." From page 261, Lind notes the exodus of over two million Southern whites who left Appalachia for Northern cites between 1940 and 1970. They, along with poor black migrants, formed a permanent underclass in cities like Detroit, Chicago, St. Louis and Cleveland. Southern business and commercial fortunes did not begin to revive until the late 1950s and early 1960s, nearly a century after Appomattox.