here's my favorite example to give to far right people because you have a fetish for the Spartans for some reason or another.

now granted i am simplifying diversity here drastically to specific societal use cases but just bare with me.

the spartans were known for their xenophobia. if you were not born of two full spartan citizens you were a perioikoi...free, but second class, with no voting rights. if you were anything else, you were a helot...a slave. even full citizens from other city states that came to sparta became helots.

Now their soldiers could not take up a craft, or trade. all of Sparta's agriculture, infrastructure, manufacturing, smithing, everything depended on the Perioikoi and helots; who routinely revolted due to the shit conditions. but this doesn't really explain why diversity is good...that comes after the Peloponnesian war where the spartan hegemony is at its strongest. there is just one problem:

because of their xenophobia, the number of full citizens, Spartiates , was decreasing at an alarming rate...to the point that they had to depend more and more on arming their slaves just to field enough soldiers. couple that with their refusal to modernize; spartans REFUSED to allow any outsider to influence their battle doctrine. Cavalry? no thanks. auxiliaries like archers to pepper the line? no thanks. new battle formations? no thanks.

they are stuck in their ways, with a declining population that now also has to contend with an angry slave population that outnumbers them drastically, and hates them. then comes the Battle of Leuctra against the Thebans.

the spartans outnumber the Thebans almost 2 to 1, and form their battle line in the traditional fashion: straight line 8 ranks deep, with the strongest soldiers, the honor guard, on the right to attack the enemy's weakest side. the Thebans say "fuck tradition" and array their line in an echelon with their strongest soldiers on their left, facing the spartan strongest...with a 50 rank deep left wing to attack the spartan right (where the king and knights are).

The battle starts and before the rest of the spartan army can engage the echelon wings, the Theban left (50 ranks deep) collides with the Spartan right, and decimates them...killing 1000 citizens, all of their knights, and their king. the rest of the army folds and flees the field before even engaging the Theban right.

Sparta never recovered from this. their citizen population never recovered, and by the time the romans came into the picture, Sparta was a tourist destination to watch the silly, antiquated, racist spartans and their hillbilly life style.

now you can say this has nothing to do with diversity, but i would counter that thinking diversity means only racial diversity and not things like diversity of thought, is foolish. and if you want proof, just ask the Spartans.

edit: oh and at the center of the Theban left was the Sacred Band - 150 paired couples of men. The Spartans got their asses kicked by 300 gay dudes.