Sturdy class structures, although they may diminish individual opportunity, keep superior genes, when they arise, within each class. In doing so, then, they strengthen classes at every level.
High social mobility, by contrast, tends to “boil off” superior individuals, who, when they are given the opportunity to do so, move up and out — taking their genes with them. In this way every class, at every level, loses its best people to a class above it. Because the class system is not bottomless, this means that the lowest classes continuously deteriorate, while more gifted individuals cluster in the higher classes. (This latter tendency is perhaps mitigated, somewhat, by the somewhat lower likelihood of inferior higher-class individuals moving downward in class.) This necessarily increases social inequality, and therefore social tension. It also instantiates the “Peter Principle”, in that individuals will rise until they find their level of social or professional incompetence, then stay there. This leads to the presence at every level of individuals who are not naturally well-fitted members of that class. This has an entropic and disordering effect on organic hierarchies.
On the other hand, too rigid a class structure prevents the ascension of exceptional individuals, and so not only thwarts individual liberty, but also blunts the leading edge of a society’s progress and accomplishment.
So: What is the proper balance? What is it that we should be seeking to optimize?