Death as non-certainty

In older philosophy, and even in quite recent work, the so-called certainty of mortality often plays a great role, as with the Existentialism of (or with the name) Heidegger that reaches into Tolstoy's Death of Ivan Ilyich. Given the existence of credible claims to overcoming or "curing" death, e.g., from Aubrey de Grey and his fellows, how is one to hear older claims to the certainty of death as what is self evident and common sense?