Istishāb preserves a previously established state when no reliable proof of change is available. But defence (dafʿ) differs from affirmative imposition (ilzām): protecting an existing right against an unproved assertion is not the same as establishing a new claim against another party.
For a missing person, safeguarding previously owned property and awarding a new inheritance are distinct questions. In the former, the absence of proven death protects an existing state; in the latter, a new entitlement may require its own basis.
Correction: Ask whether the argument maintains a proven right or imposes a fresh duty or entitlement. A presumption of continuity may not suffice for the latter.