Mechanization and automation is a form of slave labor, but the slaves are mechanical. Sugar planters cut sugar cane with slave labor in 1770, with paid labor in 1870, and now cut cane with machines. Slave labor, paid labor and machines are interchangeable. Slaves and machines exist at a subsistence level, but paid labor, usually, lives above subsistence. In the modern world, paid labor competes with machines, and machine slaves often win. Consider the number of diesel shovels, forklifts, and backhoes in the economy, and it is obvious that if these machines did not exist there would be an enormous demand for labourers.
Consider a white man in the ante bellum South. I expect it would have been customary for whites to do skilled work, and the worst jobs would be reserved for blacks, as happened in apartheid South Africa. Some whites may have been forced to migrate to the North to find work. Replace black slaves with machines, and the economic effects are similar. Today, humans compete with machine slaves for work.
I remember the steep decline in work for laborers in the fifties and sixties as machines took those jobs. We knew at the time that the tide was coming in, and workers had to move to higher ground. Some displaced laborers moved into clerical work, but these jobs are now being taken by computers and the internet. Computers add up bank balances and send out renewal reminders for insurance.
In the sixties, any laborer who could not move up to a better job, and lived on what welfare was available at the time was regarded with disdain, but we see today that computers take back office jobs in law, conveyancing, financial planning and engineering. The novelist Nevil Shute was an aeronautical engineer before he wrote novels, and worked on the R101 airship in the thirties. He described in his autobiography spending three months with a slide rule calculating stresses at numerous points on the airship frame. Today, a computer would calculate those stresses in minutes. Even engineers feel the effects of automation.
John Keynes, and others, mainly science thinkers, foresaw a world of the future where people would live an idyllic, comfortable life supported by machine labor, with a short work week and an economy which was not growing. Today, that is called work sharing, and is the way for any advanced society of the future, together with early retirement.