That's called collective bargaining and the only way we'll end up there is strong unions. Otherwise, I promise you - most companies will not altruistically ensure their employees are taken care of; exceptions exist like Costco. Someone's going to need to bend their arm, be it a union or the government through regulation.
Also I'm not sure I understand your comment. The first sentence feels like you're devaluing the service employee that helps resolve your self-checkout issues? Then, the next paragraph you posit that we're in a transitionary period from a "legacy system" - this just isn't how I see it.
From convenience stores, to grocery, etc. there will always be the need for traditional cashiers, along side of self-checkout folks. And, personally - I believe they both deserve a dignified wage that covers their base needs (rent, car, medical, etc) without necessarily having to "bargain for a living wage."
IMO - even the most entry-level and lowly of jobs at 40hrs/week should be able to get someone by.