1. poorer people spend a much higher % of their income, whereas rich people save it.

2. there's a lot more poorer people than rich people, which unfortunately can mean they're more capable of moving prices.

This is sort of related to why the middle class thinks full employment+low inequality economy is worse. There's other middle class people to bid up prices, and service at restaurants is worse/more expensive. It's nice to be in a high-unemployment high-inequality economy, if you're employed.

I don't see how stock buybacks relate more directly to anything here than retained earnings, or other kinds of reinvestment. They're just an effect of market conditions and the stock price.