It already works, there is just a bug preventing it
U sure bout dat? When you are alone in an instance, only that instance exists for you. If the npc leaves, there is no 'container' for their persistent data to go into and if you leave, the instance and everything in it is despawned. To track every npc that high wakes from a player without an instance server dedicated to that (changing the entire net model to a persistence server), I don't see how it could be done in an efficient way. Admittedly, I'm not an elite game developer, but I've been playing enough mmos over the years to realize a few things.
The only thing I can think of in the game that has this level of persistence currently, is combat zones. Those instances seem to live longer than any other. If you think I'm missing something, please do share your understanding, I'd love to have hope!