in order to manipulate the music to slow down or speed up you have to be able to control it like a record...aka... draw the song backwards and forward... the entire essence of scratching... its the same technology. and its roots were in the needs of dj's who scratch...fap...mix....etc...
cc
Your technical knowledge is way off, but I get your point. Yes, the evolution of DJ gear IS and ALWAYS HAS BEEN driven by the demands of club and performance DJs. While the mobile will never admit it - the entire mobile DJ biz is an imitation of the nightclub and stage arena.
As for mixing with time code, that technology had been around for a long time - used principally in commercial and broadcast video editing prior to the advent of personal computers. It was extremely expensive and well beyond the access of any DJ not affiliated with a major production house. Anyone old enough to have been a video jock in the early 1980's would be familiar with SMPTE time code, jog dials, and the UMatic formats in which the music videos were edited and distributed.
You don't even see pitch control for a CD player until well into the personal computer revolution. Technics produced a CD player aimed at DJs around 1983 that featured a pitch slider much like the SL-1200 Turntable but it cost around $4,000.00 By 1985 the cost had droped by more than half. Other budget models began showing up in dual format but, their performance and reliabilty were lacking. When buffered memory became feasible the all important instant start finally made it's way into the CD players intended for DJs, and later was expanded to a range capable of shock and skip-protection.
A lot of what led us here is from advances in PC technology. Fame-sync made video DJing affordable outside of the clubs since it eliminated the need for Gen-Lock or true time base correction. Prior to that a good TBC for pre-recorded sources could run $10,000 and you'd need one for each input source.
While the MP3 dominates in what is now a software environment - that was not initially the case. The MP3 format was never intended to become a distribution format. It's accommodation in CD players shows up AFTER consumers and file sharing put it front and center in the music industry.