"Slave Age"? Slaves were kept because they were economically useful. It's not clear Humans will be economically useful in the new age.
I think they need humans to recycle money They need humans to run the green economy as the Afro-americans in the south They need humans as a hunting sport, how to make them do what we need. Let's play with them. The AI is not viable as a business model. So they will use it as a tool for their purposes as long as they can.