> Many a slave in the past (again, excluding US) was in much better situation than your average friendly neighbourhood supermarket clerk.
Unless you are using some exotic definition of enslavement, there is a fundamental difference between slave an employee. A slave is a property of his owner [0], thus slave has no agency and is completely dependent on her/his owner decisions. Employee may choose to be dependent on her/his employer to any extent she/he chooses, but it is her/his choice. You may argue that both slave and employer may end up being completely dependent on her/his owner/employer, but from the point of view of the law and society the situation is always completely different.
[0] https://en.wikipedia.org/wiki/Slavery