90% of active DJ's do not remember using equipment in the 70's? Again, using 1000 as a base number, this would mean that a very large group of those 900 somehow forgot using turntables, cd players, cassettes (and other various media players). I think the majority of 'active' 'business' DJ's are older than 30 - but according to your math, they do not remember their roots. Do they all have Alzheimers? Memory loss? If so, how do they get anything done?
The 70's had speakers, amplifiers, mixers and turntables, 8 tracks and cassettes. From the physical aspect, other than swapping turntables (and various media players) for controllers .. what's changed? Quite a few of the younger generation is into digital turntablism. Considering their interest, don't you think that somewhere along the lines, the topic of what other technology was around somehow crossed their mind? The same way you learn about older music - is the same way you can learn about older, previous or alternative technologies.
My son is 23. He grew up in the mp3 player era as well - and also knows what a cassette is and how to use it. Other children in the family range from 14-25 - and they also know what CD's, cassettes and records are.
I'm trying to figure out what you're basing your information on .. because I'm not seeing the same results with family, friends or the public, let alone 'active' professional people within this business.