1. I was in the audio servicing and repair sector for quite a lot of years, starting as a technician and working my way up into management through several repair companies. I am still involved with some of them on a technical basis. The knowledge of the difference different component types and quality can bring is not something that technicians tend to bother themselves with or get taught about. You have to move higher up the scale to engineers from university level before you start to see a jump in knowledge. But people with those kind of skills and knowledge end up in the higher end jobs as design engineers etc.

2. If something was designed with components going back years or decades, the designer is likely to have used what was available at the time and at the right price. Things like 1% metal film resistors, polystyrene capacitors etc were often avoided due to cost even though their superiority in audio circuits have been known for quite some time.
When you are doing a recap etc. the current cost of those once expensive components can make it beneficial to use the better performing parts now since production costs are not an issue in this case.

As for your comment that capacitors haven't gone through some mystical transformation: caps can be had with lower ESR etc., and the effect of different capacitor dielectrics in audio reproduction are now far better understood. New materials have been researched and developed since the seventies that have benefited audio reproduction. So capacitors are no longer what they ever were as you assume.