>idm is a fallacy. it doesn't really exist, not in the way that say, a bay b kane tune and a hype tune are unmistakably jungle.
perhaps IDM is more like a format, in the radio sense - it's a category determined by a mode of listening and an implicit demographic. A radio format can have a bunch of sounds (classic rock or modern rock - these categories cut across genres and eras) in it, but the demographic it targets is fairly defined.
same as "indie", the output of which zone now sounds completely different than in the 2000s, which sounded completely different from indie in the '80s. but the through lines and commonalities relate to sensibility and the kind of listener (and perhaps also lyrics rather than sound. indie today sounds clean and well produced and can also be well played, whereas in the 80s it meant unpolished, sloppy and rudimentary music skills. but the lyrics in all eras of indie are bookish, sensitive, non-carnal when concerned with love, mildly alienated etc)
not unrelated - when did people start talking about "uni" - "going to uni", "after uni" etc. It was not in the parlance when I was a student, i.e. early '80s. "uni" makes my skin crawl.