For starters, we have to acknowledge that capitalism isn't really a system as such. It's a set of ad hoc economic arrangements that each came into being as a result of historic evolution in many different societies, not some kind of coherent arrangement with any specific goal. If you were to have the same discussion 400 years ago, proponents of then-existing economic relationships would say, "how are you so convinced that there's a better solution than what we have right now if you can't even articulate what the replacement would be?".