I suggest reading The Selfish Gene. A system where only the selfish can thrive is shown to collapse with certainty, no matter how many times one runs the experiment.
But since you have, absolutely, capitalism only works well when there are guardrails in place. Our greatest success is creating a society based on the adherence to law and order that preserves individual freedoms. If it solely relied on capitalism, it would become possible for a person to pay someone to kill someone else. Maybe we'll get there eventually once enough safeguards are eroded, who knows?