Myelofibrosis is a chronic and progressive blood cancer that can lead to severe complications if left untreated. The average life expectancy for someone with myelofibrosis without treatment can vary, depending on several factors, such as the stage of the disease, the severity of symptoms, and the presence of other medical conditions. In some cases, myelofibrosis can be fatal, especially if it progresses to a more advanced stage or if serious complications develop.