I think you're right that many more people are in theory capable of joining the group, but not that the industry is conspiring to keep it small. If anything it's the people who have an academic history that stretches back to childhood that end up dependent on having someone to guide and grade them. Beating your own path through the wild is scary, uncertain, and often thankless, so I think you'd only want to do it if you already felt sure you were among the most motivated and dedicated members of a group.
Well, the industry is rewarding people who had the privilege to invest their childhood into one singular focus. By the time they graduate they are way above everyone else and so they can both do management and leadership while still having elite level technical skills. Most people start working on their skills in university, so I think we would see more elite engineers if companies just let them specialize over a long time, and structure the companies to allow this. Instead companies structure companies to replace engineers easily, and so the person with pre-existing technical knowledge has a huge advantage.
I don't think it's necessarily a privilege to have a childhood with a singular focus. Again, having your whole destiny picked for you leads to a kind of person who often does not lead themselves with any confidence. The trappings of success will be on them before they've ever faced a test from the engineering school of hard knocks. They'll try to lead right away (because they'll be sure they were picked to lead) but the smarter move would be to try following for a while.
I think the test is right: how long can you hold onto a single project so that you specialize more and more deeply? Every startup is an attempt to do exactly this, and most startups still fail.
I think you're right that many more people are in theory capable of joining the group, but not that the industry is conspiring to keep it small. If anything it's the people who have an academic history that stretches back to childhood that end up dependent on having someone to guide and grade them. Beating your own path through the wild is scary, uncertain, and often thankless, so I think you'd only want to do it if you already felt sure you were among the most motivated and dedicated members of a group.