You're right that the concept of evolution is nothing new, and I'm not saying that it was. What I'm saying is that it brought a completely new logic to the classification system.
The reason why I'm saying that you couldn't have possibly come up with the Darwinian theory of evolution by just classifying things, is because it's an answer to a completely different question. It's not an answer to the question of, "How should we classify species?", but it's rather, "How did those species get there? And why is there so much diversity in nature? And what's the link between those old fossilized species, and the species that exist now?", etc.
It's simply a completely different question as well as an explanation, and hence you couldn't possibly have come up with it just by classifying things. You don't need to bother with those "why's" if all you're going to be doing is to classify things.
And ironically, if you never came up with the correct explanation, then it's likely that you'd have made the classification in the wrong way. Only by coming up with the "correct" explanation of evolution by natural selection, that you can come up with new knowledge, such as gene theory and DNA theory, which further improves the classification system.