this is very true, however, shittily the automotive industry is highly competitive, compound that with parts stores spreading misinformation, and it's difficult, to say the least. So when o'reillys/pepboys/ whatever offers to "check" your check engine light for free, well that's where people expect shit for free. Same thing applies to a "pass or don't pay" smog check. Screw that, I have bills to pay, if I smog your car, i'm getting compensated.
The bright side is, the extensive education pays off a bit, local auto shops send their "trouble" cars to me, I have a reputation for being a fairly competent diagnostician (if I can humbly claim that), when it comes to troubleshooting or diagnostics, the competition weakens significantly, ANYBODY can replace auto parts... take part off, put part on. BUT only a very small percentage can actually diagnose cars, like current ramp an ignition system and tell you whats wrong solely from that, or looking at the long term fuel mapping inof, or an oxygen sensor waveform, etc, etc.
off-topic, but that's cool that you are a musician, i'm a bassist, I have an old 77 fender jazz bass, that I beat up on. Made in aug of 1977, just like me.