I don't think we need to have such a deterministic view of labor's evolution. Slavery wasn't "necessary" to get us to where it is today, and it's not necessary in the future. It's certainly cheaper and easier, but much more ethically questionable. It's not absurd to say that we could construct our economy in a way that isn't so abusive and exploitative.
And the argument that, because we all own computers, clothes, shoes, coffee pots, etc. that were made by underage / underpaid people in the 3rd world, we can't critique the system that allows for that to happen has never really held much water for me, sorry.