Sure, but in order to understand human behavior on the scale of millions of people making billions of transactions, you need to start with a simple axiom and move on from there. I think people hear the word "self-interest" and think "greed", and assume that mainstream economics must be predicted on a very cynical view of human beings. That's not the case at all. We all want to survive. We can't possibly understand the individual needs of other people outside of a small circle of people that we love and care about. None of us are omniscient. The beautiful thing about markets is that none of us need to have perfect information in order to participate. By simply buying the things we need and selling things the things we don't, prices emerge in a totally decentralized way.
If you can think of a more accurate way to describe human economic motives, I'd like to hear it.
> has been disastrous for the working and middle classes all over the developed world.
Capitalism has lifted billions out of poverty. Sure, there's a large gap between the richest and poorest, but the poorest people of the world are also materially better off than at any other point in human history. I don't see how this is a bad thing.