Has the average productivity grown that much? Sure, I can believe that. Has the productivity at the lower margin grown that much? I seriously doubt that. The value created by stocking shelves at wal-mart just doesn't really change over time. There's a lot of labor on the lower end that is a solved problem with all the productivity wrung out of it decades ago.
Unskilled laborers are just not able to capture that value from Wal-Mart because there is an abundance of unskilled labor and the average person (the laborer) has to derive income from work. They can't just say, "Puh, this isn't worth it any more!"
In addition, there's been a ton of improvements in productivity for warehouse workers. They are absolutely doing more work in the same amount of time.
The same is true at most big businesses.
That being said - at $26 an hour, there would be A LOT more pressure to eliminate more jobs.
This was true throughout the 2010s, but under full employment even a low-wage/"unskilled" worker can realistically go get a different job if they don't like the one they have. There's probably an Amazon warehouse nearby that would hire them at a higher wage, or a restaurant if they're willing to do a different kind of work.
That's the problem. The minimum wage is a legal instrument, not just an economic process. It is designed for the lower margin to be "overpaid" over what they could get from the market otherwise.
It basically says "everyone working should get at least this much of our collective production". This is what we have decided our society should work like.
Our collective productivity increased much faster than minimal wage, making the minimal wage worker much poorer by comparison.
The fact that the stores themselves are physically able in practice to enjoy this increase in productivity without increasing pay to the people stocking shelves says very little about how the productivity of stocking shelves has changed.
The idea that an individual workers productivity is derived from the total productivity of the business isn’t entirely accurate. In the case of stocking shelves, the value is mostly aesthetics and executing a marketing strategy. The value of stocking shelves is based on the value that stocked shelves provide, which doesn’t increase linearly with the productivity of a store.