No need to train replacements. I'm sure that while the labor itself is dirt-cheap, it takes time and effort and thus money to train replacement laborers as the turnover rate has to be ridiculously high. If a laborer falls ill, dies or otherwise quits, you have to spend time and money to train a replacement. If a machine breaks, you just replace it and continue with little downtime. Also machines can work 24/7/365 until failure, whereas human laborers will fail in less than a day with such a workload.
Even if the machines themselves are expensive to procure, they'll likely still end up saving money in the long run.
For added hilarity, wait for the inevitable sob stories of Chinese laborers going unemployed because they got replaced by machines.