Maybe it's because I'm in europe but extensions, developments and updates of the disco-sound were all over the charts for the entire 80's.

Did the music sound exactly like old disco? Of course not but you can say that about any genre that evolves and grows. I don't even think there was a cut-off point for the music which evolved gradually, just the genre-name... It's comparable with when grunge/''heavy alternative'' supposedly killed metal and hard rock in the early 90's; Soundgarden and Alice In Chains didn't exactly sound less metal than Bon Jovi or Def Leppard did, quite the opposite actually