The world economy absolutely was designed, just not by a single designer. It certainly isn't there by mistake.
Economic theory dating back to the late 1700's arguably were the first major attempts at retroactive design of the global economy. There've been other pockets of this happening, but not globally, more likely the monetary and debt systems of various epochs and empires.
But after retroactively observing market exchange, most classical economic theory became pretty prescriptive, i.e. it wasn't a reflection of "human nature" so much as that was an easy way to promote free market ideals.
For example, the "profit motive" has been basically disproven as something that never actually existed historically or anthropologically as a universal human driver. It was invented by economists as useful way to explain away certain aspects of their system design. This is not to say some people were not and are not motivated by profit.. sure they are. It's just not universal, like hunger, or breathing.
Markets also weren't omnipresent throughout human history until proto-economists noticed them, wrote about them, and businessmen began convincing governments to apply them by force to various communities in the 1800s. (see Karl Polanyi, the Great Transformation).
One could argue that economics is really "software design for the world economy", except you need to test and debug in production. Religious wars about better designs and features abound (fiat currency vs. gold standard; Keynesians vs. Rational Expectations / Real Business Cycles; Austrians vs. Science; etc.)
