Honestly a logical plot point as well. He is a hero-ified master and he is back to his own account-world. He would gain even more power there while still retaining the knowledge of the world; in short, nothing would stop him (except maybe from that interference points? Dunno how exactly you earn that when you're not on Earth paying gems in the game).
I'll take a bite at the bullet, though: He will refuse, but that doesn't mean the end of Niflheim's relationship. If anything, I think the bond between the two worlds can only grow after we're seeing it doesn't take much to travel between worlds (you can even "invade" other players). He will get stronger, raise to top level, confront the Mobius CEO thingy (I still don't understand if she is avatar of irl person or an NPC in the game world
), and finally return to Earth--still the og goal of his.