I doubt plumbing or electrical are any more complex now than they were 80 years ago. The principles haven't changed. What probably is more complex is dealing with permitting and inspections, which you alluded via fire code regulations.
This is sort of half true. Ancient knob and tube wiring is undeniably much, much simpler than what we have today, and you could teach anyone who ever worked with their hands to install it pretty quickly. It wasn't simple in an inspiring "simplicity is beautiful" way, but rather in an "oh my God we are all about to die in a fire" way.
Doing things correctly according to modern wiring safety code is something that requires a lot of knowledge, and a person could curse regulation and so on while learning all of that, but using all the modern tools would require a lot of knowledge regardless. There are dozens of parts in a standard electrical installation that simply didn't exist when it was just knobs and tubes.
