Yeah, there is no universe where there is enough demand for plumbers to sustain even the same order of magnitude of number of jobs for even a smaller category of knowledge work. And when you unleash millions of plumbers, the wages won't look better than McDonalds.
The market can never be big enough for plumbers to absorb even a tiny bit of knowledge workers. There is not enough plumbing to be done. Also if a lot of people do try to become plumbers, the wages will plummet similar to other jobs where availability of labour is high. Your statement will hold true if 2x more people wanted to enter plumbing, not 20x or 200x.
