Agreed about 2001. I remember hearing Dre replica beats from producers for the next 5+ years. Dre also carried his sounds with and had a damn near flawlesss 5 year run after 2001 dropped.
I think the 2001 sound aesthetique ended with Busta Rhymes The Big Bang album. After 2006 Dre was looking for a new sound but was never able to create the real magic like on his last great era. To this day every normal music fan loves the sound Dre created in his prime. You just cant deny the fact that these songs all sound as fresh as they came out in 1999 to this day. Show Still D.R.E. or Next Episode to a 14 year old or a 65 year old human being, above 90 percent of all civilized people will know the songs and also the most important thing: Really like them...
I blame it also on the Trap era which lastest waaaay to long in hip hop history. Think about it.. At latest when Soulja Boy came with that Crank That joint in 07, this type of sound took over mainstream hip hop music worldwide. Of course the alternate hip hop genres survived but had a hard time and lived in the underground in the 2010s in my opinion. Thank God its slowly fading away and we are going back to more modern boom bap orientated music with real raps and less mumbleness and autotune.