My favorite documentary I have not seen (yet), I Dream of Wires from 2013, about modular synthesizers. I know in some trailer there was a maker of modules saying something to the effect that if only people actually making music with their synths bought modules he would be out of business. Can't find that trailer now or I did not watch carefully enough now. There are a few different ones on youtube.
I’m pretty sure the person who said that quote about non-professional musicians who purchase Eurorack modules was Paul Schreiber, who passed away about a month ago.
Gonna look it up, and I’ll edit this post when I find out.
Edit 1: Didn’t find the quote from the film yet, but did find [1]this video (unedited interview from I Dream of Wires) where Paul explains how he himself is not a musician, but rather an engineer.
[1] https://youtu.be/6ixv4F4XD4Y
Edit 2: Still haven’t found it.
I have the film at home, but I’m traveling in Europe at the moment, so it is out of reach for me currently.