That would also mean most story lines would never matter in any way. When you have events that shape up every faction dynamic ingame you can't just pretend that this zone and these people, besides being part of the larger conflict, aren't affected by it. That just makes it so that either the overarching conflict loses significance or the zone itself does.
Personally I think blizzard should invest in a GUI element that essentially works like a diary and let's you pick which part of the wow timeline you want to experience, that way you could have old zones and questlines as well as new ones without having to rely on cluncky NPCs (still vastly better than nothing!!) or never actually showing any zones progressing. After killing 2 million gnoll invaders I'm rather confident that I've genocied them to extinction, how about that invasion event stops now and I can experience a post-invasion zone. From there they could implement new story lines (always coming up with new places is another issue in wow development) or leave the zone in this state and if I ever need to do something in the previous zones iteration I just dial back time.