As I said elsewhere in this post, IMO both the shape and the place of the bell-curve are probably worth discussing.

Personally, I suspect that the shape itself isn't as important as mobility, but that the shape of the curve and mobility may be related at times to different degrees. If there is insult of disparity, then surely the possibility to move from the lower strata to the higher one reduces the sting. And, due to some evolutionary oddity of our brains, a little bit of this goes a long way. IMO the US has long benefited from a worship of economic transcendence.

There's that quote (mis-attributed to Steinbeck) that pokes fun at Americans:

"Socialism never took root in America because the poor see themselves not as an exploited proletariat but as temporarily embarrassed millionaires."

It's a funny cultural observation, but when your soldiers have irrationally high morale, you win more battles.

To be clear, I agree that disparity isn't an injustice; at least I suspect that you don't count it as one. However, I do see an increase in disparity coincident with a decrease in mobility as symptom of something that won't end well.

My sense is that due to the influence of money on law, there may be a trend in the US towards systems that preserve wealth and prevent mobility at the expense of a greater economic growth. That is, the change in the shape of the bell curve is slowing its shift to the right. To me, this would be a real concern.

Here's a horrible thought: What if the hope of economic mobility is not fueled by an absolute rate of mobility, but instead by the sign of the change in that rate? What if the the shape of the bell curve doesn't matter, but the eventual shape that its current change suggests that does?