People who defend billionaires either have a vested interest, have actually bought that they're 1000x smarter than normal people, or have some (possibly vague) abstract moral position that overrules the basic idea of fairness. Often it's more than one.

Capitalism, as the term is commonly used, is poorly defined enough that you have to specify what it means here. Is it any kind of market? Is it large corporations? Is it every interaction being purely voluntary (somehow)? If you consider a big Soviet firm like Gosbank a "corporation", all three could also be socialist depending on who you ask.

Since this is .ml, for the classical Marxist definition that it's "private ownership of the means of production", the arguments are mainly against the proposed alternatives, or just that private vs. personal is hard to demarcate, and nobody wants to share a toothbrush.