Unpaid labor makes for big profits but taking proper care of your investment costs considerably more than low-paid unskilled labor from an almost unending labor pool.
The Northern capitalists grew wealth at a phenomenal rate when they began using cheap imported labor. There were no medical, clothing, housing, or food expenses. Slave owners had to take care of all these things for their slave property while their Northern counterparts simply discarded workers who had outlived their usefulness and hired another. And the cheap labor just kept on coming.
Loewen seems to think that slavery was at its peak in 1860 and would keep going for a long time to come. We can never know how soon it would have been phased out but there were already movements in the South itself to abandon the practice and some of their European markets – who had outlawed slavery before 1860 – were already putting pressure on the South to do the same.
If the markets began to close because of the practice, the South would have been forced by the economic reality and probably would have adopted the “Northern model”. The war, at least on their side, was based on economics anyway.
I have seen sensible arguments both one way or the other by rational scholars. So this is one of the myths that may or may not be a myth. Loewen’s guess is as good as anyone else’s but his opinion does not alter the possibility that it could have ceased in a relatively short while.
No one knows.