[Cyberdemon]

Not a field I would ever want to get into to get rich. I had a friend in college who started a performance/mechanics shop and wound up shutting it down a few years later.

Like mentioned, usually the guys who are successful:
-Have a niche for a few manufacturers and thats it. As mentioned the tooling, training, and computer requirements are increasingly complex for new cars and that requires a LOT of up front capital. Smaller shops which do everything can usually do it if they stay very lean, which minimizes how many cars they can move through the doors. I know owners of really successful VW tuning shops who only do VAG cars and small German stuff from time to time.
-To run a big shop often means doing the shit we hate dealerships for doing. I watched a great mechanic friend of mine leave his business because the shop grew to ~10-15 bays and in order to keep the money coming in to keep the operation running they needed to get as many nitrogen refills, air filters, and other potentially un-needed services run through as possible. It changed the dynamic of the business in order to keep it alive, and the guy who was just a good, honest hard working mechanic left to go back and start his own rural 1 man shop.
-Customers are also sometimes just assholes. The customer service portion of this job isn't something that I envy, especially with car enthusiasts (myself included) who will show up and tell you they read something on the internet, so that must be whats wrong. Sometimes they're right, but often times they just don't know what they're talking about and thats a human to human job, not a human to machine job which is what mechanics are good at fixing.