I don't know but somehow it's better that he died in 1918 than later; let me elaborate. He died as a bold new pioneer of aviation before the Luftwaffe was banned, before Germany lost the war, and before nazism and Hitler... In some ways it was better for him to die young gloriously than live to old age chained to a bed while slowly rotting from some random disease and watching the world change around you change with you having no influence at all and generally being out of touch. Many people would rather chose to die with glory and be remembered than old and forgotten. And who knows maybe Richthofen thought about this himself on his last day before retirement, the day that he died by making an uncharacteristic mistake for him.