By "trades," I was referring to those occupations requiring skilled manual labor, usually mechanical, or electrical. These are also referred to as "crafts." As a professional engineer myself, I have produced designs which have been fabricated or built by carpenters, plumbers, electricians, drywall installers, welders, steamfitters, instrument technicians, sheet metal workers, painters, and so on. These are occupations that need to be done on site, not in one location, like a factory, and often at different sites in a week, and as such are not likely to be replaced by robots.
When I was a child, San Francisco public elementary schools all had a wood shop with a teacher, or if they didn't, then they were required to send their male students to a school which did have such a shop, for training in basic carpentry and woodworking skills. When I graduated and went to a junior high school (now called middle school), the junior high school also had a wood shop, and in addition had a sheet metal shop and a print shop, where boys could be taught how to fabricate using sheet metal, and how to set type and run printing presses. Today, printing has become much more automated, virtually eliminating that occupation, but the need in society for carpenters and sheet metal workers remains. Most of our high schools at the time had shops like and auto shop or an electronics lab.
My father also had training in carpentry and woodworking as a child in elementary school. Some of the furniture he made, I still have and display. He went on to attend Cal in the 1930s and study architecture. Frank Lloyd Wright was a big influence on Bay Area architecture, and he believed that the architect should not only design the building, but also the furniture that would go into the building. My dad's woodshop training held him in good stead, because when he was a struggling young draftsman working to get the experience to get his Architect's license, he used to leave home at night, and head over to a carpenter's workshop, where they fabricated pieces of modern furniture. Over two hundred of my dad's coffee tables were sold by Gumps in San Francisco. He always said that architects who had a background in carpentry made the best designers, because they knew how to put two pieces of wood together. My dad's drawings, notes, and files are now a part of the Archives of Cal's College of Environmental Design, where students study them and work on their theses based on his work. A final note: My dad used to hire Cal graduates when he could, but he found them to be to theoretical and impractical. He much preferred hiring Cal Poly graduates because they already knew how to make a working drawing without any instruction. Cal grads from say 1950-1995 at least, had to be given lots of training in order to produce a design that carpenters could build. As a small business, my dad could not afford the time to train them.
I followed a similar path. As my dad was pretty busy, he had no time to repair anything around the home. My mom had her own toolbox, but when I became old enough, I repaired everything for them. Radios, washing machines, clocks, their automobile, power tools, a refrigerator, chairs, and tables. I had the woodshop training in school, so I learned the carpentry skills. After Cal, I joined the Cal staff at the Richmond Field Station, where I did research and advised students. A maintenance man there taught me to steamfit, to do plumbing, arc welding, oxyacetylene welding and cutting. I spent my career as an engineer, first mechanical, then civil, followed by electrical. In all those fields, I always felt I was a better engineer than I would have been if I hadn't had all that training in the trades.
Finally, I'd like to add a word about whether any of these trades are disappearing. I think demand is increasing, actually. As I wrote elsewhere, I used to do all the work on my parents' home and now on my home. When I got to old to be climbing on the roof or going into crawl spaces, I hired illegal aliens, because I could not afford professionals. I taught them how to use tools of all kinds. Now they have gone on to get steady six-day-a week jobs, and I they are more skilled than I ever was at this work. I need all sorts of work done on my home now, but I can't find workers. Try and find a carpenter, even a handyman, and they tell you to wait six months. Many seniors need work on their homes. I would add arborists to this. I have big trees, and I have to wait months for an arborist to do some pruning, and the prices are many thousands of dollars. I need landscaping work, not just "mow, blow, and go", but people who know trees, shrubs and plants and can do pruning and pull weeds. So far I can find no one. There are no more illegals on streetcorners looking for work. In this town, they already all have good jobs.