I wish people would be more specific with these type of bumper sticker slogans. Like which specific type of diversity is suppose to be so beneficial to us? Because at a biological level, genetic diversity has been scientifically demonstrated to aid recovery rates of plant life within ecosystems struck with drought, disease or other adverse events, but within this context it's not clear that genetic diversity is being cited as the driving force that makes people stronger, wiser or otherwise expands our understanding of "right and wrong".
Ironically, if we try to apply the cited benefits of genetic diversity to human populations, social perceptions of 'race' become obsolete and arbitrary, simply because there is not always a direct correlation between phenotypical variation (i.e. skin color) and genetic variation. Thus if genetic diversity is cited as beneficial (i.e. such as an ecosystem's response to a drought), and phenotypical variation has little to do with genetic variation, then in the interests of genetic diversity it would be possible to have populations that are exclusively black or exclusively white, a scenario that seemingly contradicts conventional notions of "diversity".
Of course, perhaps the OP isn't referring to genetic diversity so much as religious, political or phenotypical diversity, at which point biological science suddenly offers no indication that diversity would offer any measurable benefit to the population. Regardless, 'diversity' is still so precious that certain universities or certain companies might go out of their way to favor one physical trait over another, thus creating tangible instances of inequality, openly exercised in the name of a vague social concept with no clear definition or measurable benefit.
I expect that you can provide an example of when the deliberate pursuit of ethnic and mono-cultural separatism or purity has had beneficial consequences.
All I can think of is how elites have exploited conjured fears of difference and diversity, the familiar disappearing or being jeopardised by the rival, for their own benefit, causing segregation, by design or by default, and all the crimes that go alongside that.
Your post, at least implicitly, suggests that mono-ethnicity or mono-culture is manifestly and measurably beneficent, which it is not, and the burden of proof for seeking to change that is with those preferring an alternative, which it is not. Especially when it is a case of "objectively is" rather than "preferably ought to be" anyway.
I'll be honest with you here, I dont generally think that a platform should be afforded such thinking AT ALL, its just so wrong. Its right up there with holocaust denial, creepy debating of the age of consent and the like. So its unlikely that I'm going to post much more on the topic. I am aware that posts like this are all part of the attempts to revive a lot of thinking which people were hopeful had been eclipsed or present it as the only possible alternative to what progressive politics presently has deteriorated into. I just dont want to play those games.
Before anyone says anything about it, while I dont think they are equivocal, I do reject ALL the entho-nationalisms I dont care if its aryanism, zionism, pan-africanism or any other sort. Bertrand Russell actually wrote a great fictional story about this once, I think it was collected into a book called The Devil in Suburbia or something like that, which featured the rise and fall of bizarre civilisations based upon ethnic wars and ethnocentric religions (his real target was religion), he was paraphrasing the socialists ideas about class struggles but he basically said that so long as this cycle goes on real human history cant begin and I think he was right.