I don't care if they end up in the same place they started -- that's got nothing to do with the discussion. I don't understand why you think this has anything to do with social mobility.

As long as they don't end up at my place stealing or breaking shit or generally holding other productive people back, they aren't a problem to me, and really aren't relevant to the discussion. The "bottom quintile" was generally not a problem for anybody in my town.