If Iceland was colonized by Scandinavians, why are there still Scandinavians?
Yes, and another example that may be closer to home for many creationists would be:
"If the people in Boston are descended from the British, how come there are still British people over in Britan?"
Not bad. You got the gist of the discussion, that only some of parent species must evolve (the dirt example is useful too). Of course, your example is written to make fun of the idea as you know - hence your exaggeration of the dangers and lack of mention of the benefits. You also mention things as if early humans planned them: "thought", "learned" etc, which is Lamarckian. But hey, I like having fun with ideas too, so I enjoyed your post.