They can't go on with it forever, they would have to abolish slavery someday. But when would that be? when do you think the CSA, had it won the civil war, would abolish slavery? I think they would have abolished it either in the late 19th century or the early 20th, but would continue on as an apertheid country(like South Africa) until at least the 1980's(perhaps become equal at the same time South Africa abolished Apartheid IOTL)