[intjk]

Is "ability", or market value, static over time? I don't think it is, wrt both an individual's ability to change learn and grow (short term), as well as natural changes in the market (long term).

This doesn't disprove your statement--social mobility can still go to zero--but I think it's something that people can, should, and do strive to make as large as possible, be it individually, through legislation, etc.