I think this Saturday Morning Breakfast Cereal comic is pretty good. Nicely shows the flaw in the argument that allowing human enhancement will reduce human diversity, and how it might very well increase it.
Any enhancements that have no downsides would naturally be the most popular, but many other enhancements would have at least some trade-offs (the monetary cost being one of them). Being very muscular might be something that some parents would choose for their children, but others might want them to be elite endurance athletes. So just like many people have the same iPhone but have very different choices of color and fancy patterned cases, many enhancements will be the same but they will have a few optional extras that increase diversity.