Distinguishing an individual from a kind is essential. Two wooden boards may share bodily extension yet differ in their specific dimensions, states, and individual identities.
In the philosophical account, the disappearance of one individual form does not necessarily end the entire series of forms of that kind. For example, melting changes a particular piece of ice. It does not, on its own, prove that matter has always existed.
The coming and going of individuals and the possible continuity of a kind are distinct questions. They must remain separate to represent the argument about the world's eternity correctly.