I have some similar feelings to AI here. The advantages of self-checkout are only present because we've made existing systems worse to the point where something not-great is now a better option.
In the 1980s, a grocery lane would have a check-out clerk and a bagger. Sometimes two baggers. You would put your items on the belt and they'd be scanned and bagged (well, so that eggs weren't crushed, freezer items were together, etc) about as fast as you took them out of the cart.
By lowering wages and reducing staff, this got worse. Now the check-out clerk is poorly-trained and probably quitting/being laid off soon and the bagger has been laid off. Mysteriously, the human-checkout line is now kind of slow and frustrating!
Having individual customers do the check-out themselves is even worse (so frequently, someone gets the machine stuck yelling about putting items in the bagging area, or just stares vacantly at the screen), but by parallelizing the process it comes out to usually slightly faster.
I think "support the working class" doesn't have to mean "do whatever they say". We're all generally propagandized into oblivion. Like, the US working class has been concerned about immigrants and trans women in sports but it's not pro-worker to go after those things.