Forty years ago I began my career repairing industrial electronics equipment. Right out of tech school with a wife and a kid on the way, I worked on precision measurement equipment by day, and played rock 'n' roll and fixed musical equipment on nights and weekends. My employer was mismanaged by a group of salesmen who knew nothing about any part of the industrial equipment I installed and repaired. One of our two vendors refused to provide any schematics or service information for their products and they were also incapable of repairing their units in less than a month. Our customers expected one-day service and went ballistic over a day of downtime. So I spent a few evenings looking up component part numbers, tracing circuit boards and drawing a set of schematics in order to troubleshoot these products.
A year later I was offered a slightly better position with a competitor and I took my service information with me. My new boss was a technical guy who had clout with management. When the uncooperative vendor called on our sales department, my boss told them that we wouldn't install or support any product that came without factory-supplied service information. The vendor refused to provide that documentation and our company told them to take a hike. A few weeks later a box of schematics, installation documents and calibration information appeared in the mail. We did end up working on the equipment, but that vendor was always our sales department's last choice in any installation.
I quit tracing circuits in order to build my own schematics about 25 years ago, but it's not that knotty a process. Anyone who really wants to know how a piece of gear is built can figure it out. Analog circuit designs in particular are just not that sophisticated.