They might all be NPCs, as a matter of fact, like Ryll said up there. Which wouldn't be so different from what I said, only copying, not moving. That would explain everything perfectly, including the fact we haven't seen anybody disappear, which happened in SAO a lot. In the end if this game had tens of thousands of players all over the world, or is it that much in Japan alone and more abroad, there's no way great many wouldn't be unplugged by force in RL or simply die in RL. But if they are all digital clones of the players' consciousnesses, then it's natural they don't leave the game ever, no matter what happens in the game or outside of the game, provided the servers keep running. There's also no way they could realise that by observation alone. Somebody would need to think about it abstractly and accept it without proof, like a religion. It could be quite a mental shock to great many, though.