Chronic Idiopathic Thrombocytopenic Purpura (ITP): Given the long-standing nature of the thrombocytopenia without symptoms, and a relatively stable platelet count, ITP is a strong consideration. However, the presence of leukopenia (low WBC count) is not typical for ITP, which usually presents with isolated thrombocytopenia.

Myelodysplastic Syndrome (MDS): This condition can present with cytopenias (low blood cell counts) including thrombocytopenia and leukopenia. The long duration of these findings in an elderly patient makes MDS a plausible diagnosis.
Chronic Lymphocytic Leukemia (CLL): Although CLL typically presents with lymphocytosis (high lymphocyte count), some variants can have a normal or low WBC count. The prolonged nature of the patient's findings could be consistent with an indolent form of CLL.
Hypersplenism: This condition, often due to splenic enlargement, can lead to sequestration and removal of blood cells, resulting in cytopenias. However, it would typically be associated with other symptoms or signs of splenomegaly.

HIV Infection: HIV can cause a variety of hematologic abnormalities, including thrombocytopenia and leukopenia. Given the potential severity of untreated HIV, it's crucial to consider and rule out this diagnosis.
Systemic Lupus Erythematosus (SLE): SLE can present with cytopenias among other systemic symptoms. Although the patient is asymptomatic, SLE can have a varied presentation, and missing this diagnosis could have significant implications.
Large Granular Lymphocytic (LGL) Leukemia: This rare leukemia can present with cytopenias due to immune-mediated destruction of blood cells. It's essential to consider due to its potential for significant morbidity if left untreated.

Aplastic Anemia: Although typically presents with pancytopenia (low counts of all blood cell types), some forms can have a more limited presentation. The long-standing nature of the patient's cytopenias without significant symptoms makes this less likely but still a consideration.
Paroxysmal Nocturnal Hemoglobinuria (PNH): A rare, acquired, life-threatening disease of the blood characterized by the destruction of red blood cells, bone marrow failure, and the potential for thrombotic events. PNH could present with cytopenias, but it's less likely given the patient's asymptomatic status and the specific pattern of cytopenias.
Felty's Syndrome: A rare condition characterized by rheumatoid arthritis, splenomegaly, and neutropenia (low neutrophil count), which could potentially explain the leukopenia. However, the absence of symptoms of rheumatoid arthritis or splenomegaly makes this diagnosis less likely.