I have an elderly family member whose platelet count went down to below 10x10^9/l but he recovered. could you please give me a reference to an article in a reputable medical journal that explains why such a low plt count is dangerous?

Cant cite a paper: since what you ask is basic physiology and something implicitly understood by the readers of medical journals. Platelets are fragments of very large cells in the marrow called megakaryocytes and function as physical building material to plug the hole when there is active bleeding. Without them you could bleed to death from even a minor cut. At a level of 10 there are effectively no platelets.