why didn't I say working class instead of slave? I don't think most people have in mind the same meaning of "working class" as I intended, while the term slave immediately communicates the situation and the reasoning of the meme

Sure, my communication could have been more specific, but then it would have been more verbose as well. This is just how we use language, to communicate effectively. I don't want to dismiss your point that being too glib or broad with our language can be offensive to some, but I also think the TJ Maxx worker is closer to that literal slave in the field than you think. To me, solidarity for the working class and cooperation is preferable and pragmatically more likely to achieve political successes than gatekeeping suffering.