> Why, so you can avoid having to spend more time doing what you specialized in?
Yes. Part of enjoying life -- at least for most people -- is taking part in challenges and endeavors in a wide range of activities. If you can work for 4 hours a week and maintain a high income, this would fit the bill but probably not realistic for most people (do you have suggestions besides managing a portfolio of capital, or free-lancing?). MMM is not saying we should do everything ourselves, but to be smart about what we do save have more time for other activities. This is how humans have been biologically desired over millions of years -- to be adaptable and have a wide range of skills. Legs for running, hands for climbing and building and fixing physical objects, noses for smelling. Skills that are unnecessary for simply writing code but useful to use. The flywheel sounds a bit too mechanical and less in tune with our biological needs.
"A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects."
— Robert Heinlein, Time Enough for Love