It was a significant contributing factor. Consider, for example, the American Civil War - the outcome would have been very different if not for the fact that the North's non-slave industrial economy worked.
This is in fact a highly interesting debate. Other than economic circumstances, I would argue that we have little reason to believe that human empathy changes substantially.
Slavery went away for farming and balancing reasons, child labour for productivity reasons and working women came for demand reasons (e.g. war).
