Self interest and greed are not the same thing, though of course they can overlap. Self interest is a framework for people to adopt. Greed is a characteristic of unethical people. Can capitalism be an ethical project while unethical people simultaneously adopt it as their own? Of course, just as with Marxism as an ethical project which saw unethical people adopt it as their own.
Regarding the fantasy, I not saying that it was some sort of fabrication or a la-la land flight of fancy, merely that it was developed by way of exploring the unconscious associations which allowed capitalism to function, back in the mid-1800s - ie, Marx analyzed capitalist relations, and then proposed a reordering of those relations (labor, scarcity, production, all the terms that constitute Marxist analysis, the discourse itself). The reordering called Marxism is thus ultimately tethered to the unconscious building blocks of capitalism itself. And, for it's part, capitalism appropriated Marxist discourse in many cases and thus, yes, pure capitalism hasn't existed because its a mixed market economy. But when I say capitalism I mean the actually existing system rather than some Ideal.