That's a different kind of plumbing than what I was talking about, and I only simplified it for the case of an example. There's nothing preventing people to organize into a complex team of plumbing specialists, just like they would learn advanced engineering or medicine, and build chemical plants and factories or hospitals. Again, it would still be driven by need. If your need drives you to be an individual plumber for suburban homes, it can also drive you to be part of a specialist plumbing team in a large city.