With capitalism proper being a decentralized system, the alternatives are all forms of central planning, so it is reasonable to assume that central planning is what is being referred to when someone says capitalism is a failed experiment. Rejecting decentralized coordination leaves planning as the remaining category.

If you think anything I wrote is a non sequitur, that simply shows you are missing the conceptual scaffolding behind the argument. The purpose of any economic system is to allocate scarce resources among humans who have effectively unlimited desires. Scarcity is the starting condition. Allocation is the problem. Economic systems are different strategies for dealing with that problem.

That scarcity does not come from capitalism or from any human institution. It comes from physical existence itself. There is finite matter, finite energy, finite space, and finite time. Scarcity existed long before humans ever appeared, and it will exist long after. Showing that scarcity is universal rather than human-created is not a tangent. It directly addresses the foundation that all economic systems must operate on. No system gets to escape trade-offs, because the trade-offs are not created by the system. The system exists because of them.

The system we live under that forces people to live with limited means is reality. Capitalism is one method of dealing with those limited means. Central planning is another method. Both are attempts to solve the same basic coordination problem. One distributes decisions through prices. The other concentrates decisions in administrative structures. Neither one abolishes scarcity. They only differ in how they respond to it.