You've fell into a trap. That's a narrative that was pushed by the slave proponents and has lived on in many circles. it has a veneer of reasonableness that sounds very logical. That argument was used to both justify slavery and to ensure that the people on the second to last rung on the ladder (the Irish) hated the blacks and would failure to see their common ground.
In fact people abuse their cars and slaveowners abused slaves. Both by ignoring human needs, committing acts of brutality, including rape, beating and execution. Following your car analogy, slaves were also leased out, and their renters treated them like rental cars -- even worse.
Like every business decision, chattel life had ROI. Eventually old milking cows are sent off to slaughter. In the case of chattel slavery, read about the Caribbean slave economy, you'll read of high "turnover" among slaves in the sugarcane. I'll assure you they weren't going to school and getting better slave gigs -- "turnover" was mostly from death.
There are good, first person accounts of this world. Read them. The first link is from interviews with former slaves in the 1930s. The second is the tale of a slave auction and the third is the related, very readable acccount of the wife of a plantation owner.
Born in Slavery: Slave Narratives from the Federal Writers' Project, 1936-1938:
http://www.loc.gov/teachers/classroommaterials/connections/n...
The Weeping Time
https://www.theatlantic.com/business/archive/2014/07/the-wee...
Fanny Kemble diaries:
https://archive.org/details/journalofresidenceongeorgianplan...