The strongest candidate I have seen suggested is bone marrow cancer, not that uncommon in older people, it would explain the mobility issues, as it can cause severe pain. In the very elderly treatment is mostly fairly low key, top up transfusions, and some milder chemotherapy. Eventually there is no positive response to treatment and sufferers die soon after. Sometimes due to a bleed ( low platelets reduce clotting,) overwhelming infection ( low white cells or only immature ones) or the effects of low haemaglobin. But we will never know for certain and in the end it really doesn't matter. An elderly lady dies at home with her family. And that's the best way to go.