>It seems self-evident that a biology that didn't synthesize carbohydrates from carbon dioxide would be outcompeted by those that did.

Their shortsightedness poisoned the Earth with the waste products of their energy production, changing the world's climate, and driving many of them to extinction. It also skewed the efficiency numbers towards organisms that didn't need to sit around synthesizing carbohydrates when there was so many free carbohydrates just sitting around for anything able to digest them.