Can you go into more details about the question?

Slavery is not the only form of labor, it's possible to have non-slavery labor with sustenance wage levels.

I'm going to take a stab at your question, but please let me know if I'm misunderstanding something.

The thing about this question is that it relies on one fundamental assumption: that slaves, in general, are economically a net benefit to the society (no statement being made on the morality of the situation).

The thing is, this fundamental assumption is heavily debated by economic historians. There are plenty of issues and cases to be made both on the pros and cons (economically) for slavery - on one hand, you can free labor is awesome, but on the other hand, there are definite disadvantages of having an entire class of economic agents unable to participate in the local economy (more information here.).

Furthermore, other economies were able to directly compete, through the use of technological advantage as well as development of complimentary industries, such as capital financing and/or food product development.