What is stopping the workers from learning some useful skill and bringing more economic worth to their jobs as opposed to a skill that will be automated 5 years down the line?
Well it's a nice hypothetical, but has very little support historically. In fact the trend is completely opposite; Amazon swoops in and other businesses fold.
The theoretical thing stopping it is simply the initial capital costs and (more importantly) the embodied body of knowledge, gained over a long period of time of "how best to win at logistics".
One does not simply hang their tile and beat Amazon at <price, service, convenience>, there's a lot of effort that goes into making Amazon as efficient as it is that cannot be simply duplicated by a random VC-backed startup. It is for that reason that the old libertarian trope about "just wait for some entrepreneur to start a better competitor" is so out-of-touch... people are going to be waiting a long time, but they need to eat today.
But then, the "competitor" or "job provider" need not be in the same space as Amazon. They can be restaurents, transportation companies, anything.
However, you will quickly realize that running a business of any sort is hard, full of unpredictable variables and very risky. The people who do it, as well as the people who invest in such companies should not be penalized by legislating a solution that would transfer wealth.
Eventually, if the cost of the employee is more than the value that the employee is providing, then the economy will move towards automating such jobs. And Amazon-type jobs are going to be very easy to automate.
When I first moved to London 10 years ago I watched as Starbucks moved aggressively into the area of London in which I was working. They decimated smaller outlets and put people out of work. I literally saw people who had previously worked at other establishments working at Starbucks - almost certainly for for less pay.
As a thought experiment, imagine that it's taken to it's full conclusion. Amazon is all that is left, all other shops and distributors are shut down. Now you have a large mass of incredibly unhappy people who have been forced into the position of working for minimum wage by power structures outside of their control. People with barely the means to survive. They don't enjoy any of the luxuries many of us have today. There's no way out for them, and there's probably no way out for their children. What happens when something triggers that group of individuals to revolt? It wouldn't be the first time that's happened in England [0].
Don't get me wrong, I think Amazon are an incredible operation. Possibly the finest logistics organisation that's ever existed - physically and increasingly virtually too.
The question is, what is the cost to society, especially in lower socio-economic areas? I think we need to look very critically on anything that clearly widens the wealth divide. Even more so when unethical means are used to shift money from broader society to those organisations in the first place [1].
In a world without Amazon maybe many of these "people who tend to be rejected everywhere else" would have gainful employment and the rest of us would just have to pay and extra few quid for the tat that we don't even need in the first place.