Hi, actually studied business administration. DEI is good for business as it gives diverse perspectives. Allowing a company to better serve a diverse base of customers and to better facilitate global trade in a diverse world.
Which is exactly what Costco stated in their response to their pearl clutching/snowflake shareholders.
Also, you're strawmanning hard in your post. No one is forcing Costco to do DEI. They're choosing to for all the reasons I outlined. Choosing not to is either based on racism/patriarchy or kowtowing to the Nazis who took over the government in hopes to curry favor. It's not based in economic principles or best business practices.
They voted to hurt me. If they hurt themselves in the process, I'm allowed to enjoy it.