In the Middle Ages, while it was definitely better to be a member of the nobility than to be a serf, noble families didn’t have it easy. They disappeared all the time—so much that historians at one point estimated the half-life of nobility.
Members of the nobility (in fiction and occasionally in life) give the impression that they have been lords and ladies since time immemorial. That’s not the case. Noble families rise and fall very regularly. Some of the falls are the natural result of infertility or, in communities with a religious bent, abstinence. More spectacular are the demises of the houses which run afoul of the monarch, or occasionally the people, and get systematically exterminated.
Some houses don’t so much fall as sink. Even when the class system was in full swing, it wasn’t as unchanging as it was made out to be. The climb from the peasant class up into the nobility took generations, but it could be done. The decline from nobility to minor prominence and then down into poverty also happened.
This happened so often that historians could look at cultures around the world and estimate the generational cycle up from insignificance and back down again. In both Europe and China, the average family would rise up and sink back down in four to six generations. The Roman Republic, between 500 BC and 27 BC, did better, as a little under one third of its top fifty families were still on top after five hundred years. But in Medieval Europe, out of any given group of noble families, fifty percent would be eliminated one way or another by the next century. Like radioactive elements, noble clans can have a known half-life.