I feel like your argument doesn't fully disprove OP's claim.

OP mentioned specifically race and gender as "superficial" categories. You say that origin, sex and culture lead to different views. I agree with you, but I would say that origin and culture are specifically profound differences. I would also say that indeed people from different race and genders, but same origin, culture (and class status, I would add) can offer way less diversity than people from same race and gender but different origin, culture and class status.

The fact is, given history of our society, there is quite a good correlation between people having different race and gender and people having different origin, culture and class status. It is not guaranteed, but it's a decent proxy, so I would say that until society is different, it's still going to be the preferred option to have diversity via such proxy. In abstract though I agree with OP, if one day gender equality is finally achieved and society bridged the gap left by colonialism, racial exploitation etc., categories such as race and gender are not going to be providing diversity.

Completely different matter is what you mentioned about having to take decisions that affect other categories of people. Here I fully agree with you, but I guess it's a separate discussion and one that should be focused on the categories of people who are going to be impacted (which is not listed to gender and race, obviously. Say rural people, disable people, etc.).