That's called stubborn idiocy, if he had admitted his defeat he could have done something for the Sith e.g. what Freedon Nadd did, impart his knowledge to a worthy successor. But he was too absorbed in his lust personal power to see the bigger picture. That's why he failed in life and that's why he failed in death - Bane would not have been impressed. Bane being a real Sith Lord, he realised is own existence was merely a component in a greater design, as such he actually achieved something. What did Kun achieve? Nada.
Nadd pretty much did the same thing, the only difference is the Jedi didn't create the wall of light to seal him off. Kun was facing thousands of Jedi at that time, he didn't have time to leave any descendents. He did leave more than Malak, Malak's empire was pretty much in chaos and crumbling before the Sith Triumvirate took over and Kun almost succeed to reemerge 4000 years later.