2 Sivan (Korach)

Why does Moshe say (Bamidbar 16,29) if Korach and his followers aren't killed miraculously then he's a fake Navi? Maybe they'll do Teshuva and live! How can he risk his whole credibility on the fact that they won't do Teshuva. (We see doing Teshuva helps because Korach's sons did Teshuva and weren't killed.)