[dipukqpo] Keep cheap services performed by lower classes

A standard theory of oppression states that the upper classes prevent upward social mobility of the lower classes in order to artificially maintain a large supply (and consequently low prices) of lower class labor to perform services for them.  Of special note: enlisted military.  Is this theory economically consistent?

In the absence of social classes, there would still be demand for the services currently performed by the lower classes, and the market would settle on a price for such services.  The price would probably still be quite low.

If the theory is true, what mechanisms keep such social structures in place?  While a vast secret conspiracy among the rich and powerful makes for a fun story, I suspect more organic and distributed mechanisms cause it.