Economically efficient for whom?
Again, you've missed the fundamental point here. Cotton was always economically efficient whether it was grown by slaves or technically free people. It made incredible amounts of money, the point is that slaves did not see any return on the incredible fortunes they made for other people.
After the civil war, cotton growers did not receive much in the way of compensation for their efforts. Sharecroppers could sell the cotton they produced, for example, but they had to pay a heavy price for their tenancy and, if they couldn't, their harvest would be taken as collateral and they would be left destitute. Many smaller farmers (including, for the first time, white farmers) switched to growing cotton because after the civil war many people ended up in debt. Predatory lenders would buy these debts with the condition, again, that a farmer's harvest act as collateral, leaving them destitute and without a livelihood if they failed to meet payments and resulting in effective, inescapable wage slavery. The reason these methods were "at least as economically efficient" was because they were not significantly more equitable than slavery. They still overwhelmingly benefited plantation owners or wealthy people who were able to continue to exploit the lasting economic inequality they had created under slavery.
The practical effect of racist violence has often been the impoverishment of black communities. Racist violence often involved the deliberate destruction or theft of property, or driving people out of communities where they had previously been able to make a living. It doesn't ultimately matter whether that was the primary intention (and I would argue that often it was), it was nonetheless the effect.
I mean, it's obvious but bears saying again.
Free speech is when I'm winning the debate.
Cancel culture is when I'm losing the debate.
