Free market economies routinely bury slave based ones. If slavery was more profitable, it would be the other way around.
Modern human trafficking is prevalent in industries that are themselves illegal, so there isn't free market competition.
The problem with slave labor is considerable expense is necessary to guard them, and the slaves not only do as little as possible but they will actively sabotage the work.
We, collectively as a species, managed to find a path in which slavery was no longer necessary - for the first time in human history.
Yes, the cost was huge - that's why stupid things like the Fugitive Slave Law and the Dred Scott Decision happened, leading to the failure of Whiggish Moderation and, well, you know the rest...
Not so much. See "Time On the Cross". Very profitable, cotton slave plantationing was - it was the feedstock to the massive growth in British textiles, the Silicon Valley of its day.
What drove slavery into the ... juggernaut it became was more that financial panics in the US ( and also even more globally ) destroyed so much cash value that slaves were all that was left of value when the tide went out.
I recommend the horribly-titled "A Nation of Deadbeats". It's narrative history, but wow its a good story. And at the same time, I bet financialization is completely critical to having the sort of standard of living we have now.
Child labor only became an issue when adult labor began to be idled.
More interestingly, society decided that slavery was wrong after it became unprofitable. The same goes for child labor.