That's such a fallacy. I can remove colour and you'll still have different religions, sexes, genders, ages, disabilities, ethnicity and sexual orientations. I can remove religion and you'll still have different sexes, genders, ages, disabilities, ethnicity and sexual orientations. I can remove ages and you'll still have different sexes, genders, religions, disabilities, ethnicity...you're not really going with that angle, are you? Let's just have a city for Christians, because they'll still be diverse. Let's just have a city of disabled people, because they'll still be diverse.
Your entire argument is invalid on principle of its purpose. Diversity encompasses a whole of singularities, not a singularity into itself.