That's a good point, however, with both upward and downward mobility being balanced, that doesn't help with the basic premise of people rising over time (i.e. the expectation that your children will be better off than you, as well as rising over your lifetime).
True - but at least you'd genuinely have a shot at your folks becoming part of the upper crust at some point. You gotta distinguish between lamenting the dumbness of relying on social mobility happening just magically and lamenting the dumbness of the worrying expectation that the economy is going to return to a '50s-like boom any way now. They're separate dumbnesses, is what I'm saying.
E: Also you have cutoffs at the top and bottom, which complicates the analysis. Come to think of it, modelling society as a vertical tube full of water might not be an altogether bad idea... Discussions of the social impact of things would then be equivalent to arguing over how the temperature curve along the length of the tube would change if we rearranged the heating elements.