Work is forced under capitalism. See, there are these things like "food" and "shelter" that cost money. You sorta need those to live. So, you either work, or you die. There's welfare programs, but they're kept minimal, restrictive and inflexible by design, so as not to compromise capitalist coercion.

You're mistaken that capitalism and the free market are one and the same, or that capitalism is the default economic system. Neither is accurate. Capitalism is a recent invention. At the most generous, capitalism started in the 1600s with Dutch joint stock trade companies. More realistically, it evolved from mercantilism when the industrial revolution happened.

The free market was coined by a dude named Adam Smith as a critique of mercantilism, the then-dominant school of economic thought. What he meant by it was essentially free trade between nations and their colonies. Back then, it was standard practice to ban your colonies from trading with people other than you, you see.

But the modern sense of the free market, as you probably mean it, is businesses making profit oriented decisions and in some way benefiting the whole. Depending on the strain of socialism, this system might remain intact. Syndicalism, for example, still has profit focused businesses, but they're worker owned cooperatives that govern themselves through democratic means.

And I don't know about whatever socialist monolith you're referring to, but as far as I'm concerned there is a natural economic system. It's called the barter system, and there's some damn good reasons why we don't use it anymore..