The Kokiri only die when murdered. If they leave the forest, as evidenced by Mido and the others celebrating (and Saria having to live in the Sacred Realm for the rest of her life), they stay the way they are. Even if the Deku Tree dies, they stay the same, yet evil does pervade the forest. Now, the only real Kokiri we see that is dead is Fado in TWW, but Ganon attacked him and stole his soul. Anyway, as for the Kokiri "evolving" into the Koroks, I think that came about because they lived away from the Great Tree. They used to live in the Forbidden Woods, and that was a ways away from the Tree, and since the Deku Tree's magic couldn't prevent the evil from entering the Forbidden Woods, it makes sense that his magic doesn't prevent them from devolving into the Koroks.