The Old World is about as dead as it's possible to be.
For a start, the Skaven blew up the moon and a good chunk of the earth along with it.
Every single elf is dead.
The Chaos gods literally walked out of the warp and killed everything after Archaon opened that warp gate below Middenheim.
GW was really trying to hammer home the fact that the Old World is dead.
But, with AOS, any (every?) hero or villian from the Old World can be reincarnated as one of Sigmars new super soldiers or as an agent of Chaos.
So while the Old World itself is no more, the characters themselves may very well reappear in new story arcs.
If you read the very first AOS book, you'll notice a good chunk of time is spent explaining that Sigmar's new soldiers can almost remember their previous lives, but not quite.
I think BL purposely kept it as ambiguous as possible in case they felt like bringing some Old World characters back.