Let us imagine that Khan wins in TWOK.
He gets his hands on the Reliant and the Genesis Device.
He gets lucky in the Mutara battle and destroys the Enterprise, killing Kirk. Khan wins final vengeance.
Then what? What is Khan's plan? What does he do next?
Announce to the Federation his possession of the Genesis Device? Does he threaten the galaxy? For what? What would be his demands?
Would he be satisfied with defeating Kirk and escaping exile and just go off to find a new world for himself?
Does he return to Earth? To do what?
Would it just be a case of Starfleet then sending a battle force to stop him and it's over for Khan anyway?
What are the things you can imagine?