Do we know they are dead? Some facts (subanon vet here)
Diesel boats like the ARG sub are hell to track (they run very quiet on battery power, submerged)
It is actually difficult for a accident (casualty) to sink an operational sub, they are designed and built to emergency surface.
Only a few casualties would do it: flooding, 02 generator explosion or major steam line rupture (less a risk on diesel boats).
Finding a sunk sub, especially a diesel, is not hard.
My guess is she is missing but not sunk.