Evolutionary design doesn't just work, it's the only kind of design there is. Design is not done in a vacuum. We fool ourselves when we write software, thinking that we're creating something ex-nihilo, plopping it down in Eden and declaring that it is good.
You're getting at the difference between top-down and bottom-up. Evolution is bottom up, starting with what exists and looking to improve. Top-down is starting with what you want to exist and moving things in that direction.
You need both. If all you do is evolutionary, then you lose the sense of why you're even there in the first place. If all you do is top-down, then you'll get deep into yak-shaving territory before you know it.
