You have no clue what you are talking about. Of course you can run a single instance on your computer with 30 NPCs around you fighting them. But i am talking about the WHOLE universe!!! Millions of NPCs, millions of markets, millions of events ...
Then you wont get million NPCs when playing offline. Easy as that.
Although i think you got that Idea of the NPCs and the universe whishfully wrong.
I doubt they are any NPCs if no player is around. When nobody is in the Wood, there are no falling trees that would do any sound.
Because why would they do it otherwise? I further believe there is not even a star system when nobody is watching it.
They are proceduraly generated if they need to exist. And are the NPCs.
Maybe the simulating things in a 30 LY Bubble around any player. But i think thats it.