The role of death in Heidegger's thinking is analogous to that of doubt in Descartes'. The Cartesian doubt is generally regarded, rightly, as a mechanism for the production of first principles. It is a means of pushing knowledge to its limits so as to discover what cannot be doubted. Those ideas which can survive even the strongest doubt can thus be considered unshakable foundations for philosophy. Doubt is thus part of the cognitive act, it is "essentially connected with the indubitable," that is with certainty.
But the basis of doubt for Descartes is man's finitude. In his third meditation, Descartes argues that if man had been able to produce the idea of an infinitely perfect being - that is, God - he would be perfect and all knowing himself and not suffer from doubt. But clearly, being finite, man cannot grasp God's infinite substance. The doubt that ultimately leads to certainty thus rests on human mortality, much as does Heidegger's meaning.
In criticizing Descartes' view of man, Heiddeger does not question the cogito as such. As we saw above, Heidegger regards man as having an understanding of Being. His argument is that taking the ego cogito as a starting point leaves the sum indeterminate.
Descartes' Cogito may be a starting point but not the end. After all that speculation in his hearth, with the melting wax, etc. he probably headed off to a hearty feast and goblet of cabernet, etc. (mayne he should have started his methodological doubt by cutting off his hand? or at least a finger) As Hobbes would have reminded him--the Ghost's apparent body must be fed.
IMO, the continental Subject is not really proven, but a matter of dogma. Perhaps elegant compared to the likes of Hobbes's naturalism but...with its own issues.