I disagree - sure a lot of the old chaos stuff is old and a bit static, but the warriors and knights are an absolutely classic warhammer look. I'd be very disappointed if they were lost.
What you could drop without a problem would be the marauders and horsemen, who are all very much showing their age. Personally, If I were in charge of remaking StD (what an unfortunate acronym) I would probably split them into two factions-in-the-same-book: Slaves To Darkness (Warriors, Knights, Chariots, Everchosen, etc) and Darkoath (Tribesmen like the Godsworn Hunt, mostly new stuff.) You can freely run the two together, but going all-in on one or the other gives you a minor boost.
Essentially my prediction for the new book is that it'll be like Gloomspite Gitz, with a big core of old stuff augmented by new things. A part of me hopes they put all of the chaos mark allegiances in there but that seems unlikely. Also I hope they centralise the rules for individual units being devoted to chaos gods: seems silly to have twenty slightly different rules on each page.