My best guess is once he'd killed Kirk and destroyed the Enterprise
, Khan would have set about building a galactic empire, using Genesis as a bargaining chip. He certainly had the know-how and the political skills to toy with the Federation, the Klingons, and the Romulans, and the threat of using Genesis on them would be an ace up his sleeve, especially if he could find a way to mass-produce the torpedoes.
The crew of the Reliant
? They would have been dead on Ceti Alpha V. Starfleet couldn't have gotten to them in time.
The universe would have been a much uglier place had Khan been victorious. I don't see that he would have been taken out in short order.