Lexicus
Deity
I mean, does it? What ultimately replaced slavery was a system of peonage that ensured the basic continuity of pre- and post-bellum Southern planter economy. It's not as if the South stopped producing tobacco, cotton or rice, after all. The ultimately significance of abolition wasn't liberating Southern blacks from the plantation, but providing the legal basis for their grandchildren to do so, and it's possible to imagine that if the United States had achieved a non-military resolution to the "slave question", it could have transitioned directly to peonage.
The expansion of convict labour after the Civil War mostly served to fill those roles previously occupied by slaves that could not easily be filled by agricultural peons, such as public works and industry. There's enough continuity in how slave labour and convict labour were use that Congress' legal identification of convict labour with slavery seems less like a technicality and more like prescience. I don't know how far that continues down to the present, but it's worth consideration.
AFAIK convict labor has never been the main or even one of the main means of surplus-extraction, though. I don't know about the immediate postwar* period but today stuff produced by convict labor represents something like one tenth of a percent of US GDP. By contrast slaves were the single most valuable capital asset in the country before the war*.
*American Civil War