Not only were they getting paid, slavery in the Roman Empire involved a bidirectional responsibility. In times of economic hardship and downturn, the owner was required to look after the slave. No kicking to the curb allowed.
Slaves were able to buy themselves out and the list of former slaves who became magnates and even politically influential at the highest level was indeed long and packed with names.
Being a slave was not considered dishonorable or lowly and the power dynamic was not based on notions of inherent ethnic based superiority. Not that the Romans weren’t plenty racist — they were. They also knew what a category error is and were not wont to induce it in themselves. Perhaps they had standards where intellectual rigor is concerned.
The ugly American version of slavery really did a number on people’s understanding of the institution. When we look at the current boardroom and C-suite “elites” and how they were often literally handed their business by the established power structures (Google and the CIA VC fund; DARPA and “lifelog” into Facebook etc), you can’t help but get the feeling that in our wage slavery society social mobility options are worse than Roman era slaves could avail themselves of.
Keen study of history is the only thing that can halt the slide of citizenry into universal and absolute ignorance. Given that Joe Schmoe no longer reads anything longer than a reel caption and even Ignatius Intellectual often maxes out at a blog post, I’d say the slide is well and truly lubed up.