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.
This idea that we can just setup a good system with good incentives falls flat on its face before we even need to consider its merits because the problem is that there are always snakes involved in the development of these systems. The most important thing for societal survival is moral character AND THEN the system itself.
So your argument is that capitalism is not about selfishness but mutual beneficial trades, as otherwise it would have collapsed by now? If you mean it is just a matter of time, every system will collapse sooner or later so that doesn't say much.
I'm not sure how capitalism crept into the discussion, since neither the article nor my comment and its parents touch on it.
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?
