You are totally wrong. Just because you get a code on a computer doesn't mean that is what is wrong. My fiancé's dad thinks like you and has spent thousands of dollars replacing parts on his truck based on codes that didn't fix anything. He did it again today on his nephews 2012 Ford Expedition which was throwing cam sensor codes on bank 2 and misfire codes for every cylinder on that same bank. They put in a new Cam sensor and fixed exactly nothing. Cam sensor code still comes back even after clearing it.Car maintenance has changed. In the old days, you had to keep tuning up the ignition and carburetors to keep things running optimally. Today, components last longer, need much less attention, and when something does need attention, a check engine light comes on. You plug in your scanner, it tells you what is wrong, and usually you change a failing component. No real diagnostic skill is needed to identify what is failing or needs attention.
My friends 2017 Ford F350 had several codes in it for a steering wheel module code, an ABS module code, and a wheel speed sensor code. I diagnosed the wheel speed sensor code first since it was the easiest to get to and determined it needed a new sensor. Replacing the sensor fixed all three codes. If I would have replaced those two other modules first based on a code, I would have cost him a crap ton of money for nothing.