When all the economic growth a country produces goes to a very small class of rich people, stupid things happen. The rich can't possibly consume enough to spend all this money, so they start casting around for something, anything, to do with all the cash they have sloshing around. And since, in an ever more unequal economy that nonetheless preaches ever rising living standards, the low-paid need payday loans to keep up and the stagnating middle class needs to borrow money on the strngth of the appreciating value of their house, that's where their money goes. It still gets spent, eventually, on things like cars and food and new furniture, because that's what middle class people mostly spend their money on, but instead of being spent directly by people who are earning it, it gets funneled downward to them via increased debt and financial legerdemain that extracts more and more money upward from poor to rich with each cycle. That's not sustainable.

Median income growth produces not just growth, but stable growth for everyone, the rich included. Top end growth, almost by definition, produces unstable, unsustainable growth. Modern economies are driven by consumer spending, and if you want consumer spending to increase consistently you have to increase consumer income. So the low/mid paid workers need a big tax cut and the wealthy need a big tax increase.