I too have had trouble answering that question for the same reason, regardless of whether its biodiversity or ethnic diversity or diversity of thought. I've sp

I too have had trouble answering that question for the same reason, regardless of whether its biodiversity or ethnic diversity or diversity of thought. I've spent a lot of time trying to formulate my reasoning and unfortunately, the easiest explanation is the most clinical and least likely to inspire: diversity is necessary (but not sufficient) for long term survival of any nontrivial system - be it a species, civilization, tribe, or business. Dynamic environments driven by random events and emergent phenomenon like weather/climate, food chains, and sentient beings are going to be chaotic and unstable in the short term. Diversity allows better suited individuals to exploit the current environment while preserving a diverse minority contingent that is far more likely to be able to adapt to the next change in the environment without a catastrophic collapse of the system. Without that minority group, the medium to long term probability of survival for the whole is zero and without the majority, the minority group wouldn't survive long enough to make it the next environmental shift. It's as close as you can get to an abstract natural law of complex systems. It applies to a company's talent pools and its ability to survive pivots or changing market conditions, an ideology's ability to survive the realities of geopolitics, a religion's ability to survive schisms and protests, and much more.