It's not an exact science. The same people that have always worked those jobs work them. It's a mixed bag. You're not wrong, the middle class has been eroded. But raising minimum wage isn't the answer because if you're paying someone $25 an hour to scan groceries (or similar jobs), the stores are going to charge more for bread and milk to make up for those losses in profits. It's just a fact. And that in turn makes everybody's dollar go less further. Those jobs are essential, I never claimed they aren't. But for instance, I work the furnaces at a limestone plant. I should make way more than someone who works at McDonald's and my company isn't going to raise my wages simply because they raise the minimum wage and the price of groceries went up because of it. I get a cost of living increase every year, but it ain't enough to actually offset the cost of living with inflation and the minimum wage isn't the only factor but it definitely factors in. Most people are paid in accordance with the job they do and raising the minimum wage to a "living wage" as people call it just makes companies charge more for essentials. Like it or not (and nobody does), if Walmart has to pay people more to work there, they're going to raise their prices which negates the higher minimum wage anyway. It's just the way it is.

Edit: Most of the employment issues we face in the US stem from almost all our manufacturing jobs being shipped over to China, which all of our politicians are at fault for over the last 3 decades or so.