> Someone convinces a whole bunch of people that maybe slavery is actually super useful sometimes? Thirteenth amendment.

Actually the thirteenth amendment explicitely allows slavery to exist in a case a whole bunch of people (maybe even yourself) think is super useful.

I was handwaving around the exception for criminals, but I concede your point: it's an oversimplification to say slavery is strictly illegal.

(One can also make some interesting arguments around the notion of the draft).