I have seen posts on social media by journalists looking to write about DEI. Instead of asking open-ended questions they were just soliciting success stories. That doesn't lead to the truth. It leads to the preferred narrative.
How did you organization create its case studies? Would it have actually written up a case study showing that the net effect was negative?
If we approach this from a different direction, can we disprove that having people from different backgrounds and perspectives who can point out things the others missed are not beneficial or provide a negative? Perhaps focusing on the claimed mechanism of benefit can shed some light on the effects.
So in general you want diversity when you want weak subordinates that are easy to control, that is the main advantage companies are really after. The other thing is if you want a group of people that holds each other accountable, since diverse groups are less trusting they are less likely to conspire, so useful on boards etc.
Also it is very easy to show how a team is hurt when normal hiring standards (GPA, other numerical prerequisites) are ignored in order to hire someone based on sex or race. I first experienced this when I sat in a classroom and worked on group projects with people who were clearly admitted via AA preference. AA was not functioning as a tiebreaker in close cases. It was used as an enormous boost, and everyone could tell.