A "Stepping stone" is the opportunity to advance upward. Most low wage jobs are simply stepping sideways.
Like I said, that's not been my experience. In the low wage jobs that I have worked, there have always been opportunities for advancement for people who were relatively decent at the job and who were willing to stick around. For example, when I worked for a major pharmacy corporation, I noticed that there were several types of low wage employees:
(1) Students and other temporary workers, who were using the job exclusively as a means of paying the bills during a season of life. These people rarely made it to higher paying or more prestigious jobs.
(2) Job hoppers, who came to work with us until a slightly better paying opportunity came along. Once again, because these workers didn't stick around very long, opportunity for advancement eluded them.
(3) Retirees, who came to work as a means of filling their time and making a few extra dollars. These workers didn't advance simply because they didn't want to.
(4) Hard workers, who simply lacked the skills necessary to hold a job with any substantial responsibility were a rare group, usually people with some sort of disability that kept them from advancing, but were quite content to have to opportunity to be allowed to do what they could.
and basically, everyone else:
(5) Future managers, shift leaders, etc., were people who worked hard, looking for opportunities to be indispensable to the store so that they would have the greatest opportunity to advance, and most of them did.
The problem with your analysis is that you are looking at all low wage workers as a homogeneous group, with each member having a similar advancement goal, which is generally frustrated by the mean old corporation that keeps them down. The reality is, however, that there are many different types of people who work low wage jobs, only some of whom are even remotely interested in advancement. And, many of those who try, actually do advance.
That's not to say that there is not a point at which the opportunity for advancement ceases without some further education. But, I don't see the problem with that. If you want to make $70,000 a year, then invest in education. By the same token, there are people who invest in education and who make the same or less than they could make in retail of service. For some of us, it's not all about the money.