Perfectly put

From Will Wilkinson's blog

Should we expect less bottom to top, number one with a bullet, mobility as an economy grows wealthier overall?

Yes. People are constantly confused by the growing gap between the rich and poor. This is good thing, not a bad thing. If the bottom is fixed, at zero income, and the top keeps going higher, you've got a bigger gap. But lots of people are better off and nobody is worse off. Similarly, if the lowest quintile is anchored by a fixed bottom, and the top is untethered and rising, the distance from the bottom to the top will increase. The distance from the bottom to the middle will increase. So it will take longer to get there. If today's middle is equivalent in real terms to yesteryear's top, people who are going from the bottom to the middle are doing no worse than people of yore who went from the bottom to the top (even if we assume, counterfactually, that there has been no change in quality of life for people at the bottom.)

We should be AIMING at a system where the middle of the middle is, say $500,000 per annum, and so the trip from the bottom of the bottom to the top of the bottom, much less to the middle of middle, is a VERY BIG trip indeed.

The original post is here. I wonder why I've never heard that arguement put that way before. It's the standard economic reasoning for a positive sum game, but that line is the best you're going to see in terms of delivery.