I agree with a lot of your points, but I've encountered many managers that you wouldn't consider "good managers".
And WRT your final paragraph...automation is going to make a job based economy a guarantee that nearly everyone is at the very bottom rung. We are already at the point where the US and India are BOTH losing jobs to increased automation, and we are still in the early days. Projections call for over half of the existing jobs to disappear within around a decade. And I don't think anyone can predict which ones will be safe. (Except upper management, and that's because they are the ones making the decisions.) As this continues it will become more and more evident that it's foolish to take on a large debt with the intention of paying it off after entering a profitable career. It isn't clear to me what is going to motivate people to study for years. (Well, I would have done it because I was fascinated by math and physics, but mine is a minority perspective, and I would have studied, albeit in a less directed and intense way, even if college had been impossible.
I agree that trade has in the past acted to suppress war. I'm not sure it's working that way in the present. Certainly simple economic arguments don't apply. The US spent more to invade Iran than the entire wealth of the country would have represented if we'd carted it off, and we didn't bother. It was politics extremely much more than economics. (I've heard it asserted that the reason for the war was that Iran started negotiating to sell oil denominated in Euros rather than Dollars. I know of no evidence either pro or con, but it's the most reasonable reason I've heard, if it's true. And since all the other reasons seem utter garbage, I tend to believe it.)
The argument that trade suppresses war has it's shining examples, but there are also many cases where it appears that war is engaged in to control trade.
Now, "Our real problem isn't that China makes t-shirts": That's not clear, or perhaps not exact to the point I was asserting. T-shirts was an example of an industry that isn't inherently centralized. Another such industry is software construction, but notice that due to the laws, customs, and business regulations of the US most software development (for profit) *IS* centralized. How things could be changed is not a subject on which I am competent to speak, but I am competent to observe the pattern. My suspicion is that this has to do with the distribution system. I have heard that to get notable promotion by or positioning within a store, you need to ... compensate ... the store owner. I used compensate where I would have liked to say bribe. I feel the process should be as illegal as other sorts of kickback, and the laws against all forms of kickback should be more rigorously enforced. Even the existing laws against bundling are either not enforced, or need to be considerably stronger.
But these are details. There are nearly endless details, it's the summation of them that tends to encourage the formation of large organizations with centralized control, not any particular one. (An exception might be the wretched and unjust Citizens United decision...though I might go back to Santa Clara County v. Southern Pacific Railroad Company, 118 US 394 (1886) and find that it, and all decisions based upon it were likewise unjust. [Or perhaps the original decision was just, but the way that it was phrased made it unjust.])