فإنه لا يستحق ميراث غيره
A missing person does not, solely through istishab, become entitled to inherit from someone else.
If a relative dies, the missing person's survival at that relevant time matters for a new inheritance claim. Presuming the prior life continues does not, by itself, establish such a new claim against other heirs.
Thus the same apparent proposition—“treat the missing person as alive”—has two different uses: protecting existing property and creating a fresh inheritance right. The first is defensive; the second requires independent proof.