Regarding the essay, the researchers selected relatively brief periods in history in countries that have had serious economic and religious problems.

I haven't read that paper so I can't comment on that but in this book author measured ethnic conflict in nations all over the world in the period from 2003-2011. He found that Ethnic diversity has 0.812 correlation with ethnic conflict. Ranging from smaller issues like racial discrimination to ethnic cleansings or separatist wars. Meaning that more diverse countries tend to have more violent conflicts.

Several millennia of cosmopolitan developments offer evidence of the importance diversity.

Yeah, how? Most development throughout history occurred in Europe which was pretty homogeneous.

If you don't like to study hisotry, you can can easily google some of the evidence about the benefits of diversity:

I am familiar with these arguments. I don't know to what degree you read these links because some of them provide literally zero sources, some of them talk about gender diversity which was found to have no impact on company performance. Some cite studies that only found benefits of intellectual diversity which was confounded by racial diversity. I suspect a lot of these findings are just correlational and not causal. You know, big companies having diverse executive boards so they wouldn't get sued by EEOC as opposed to small business owners who don't give fuck and nobody gives a fuck about. Then someone researches this it and goes "oh look, more diverse companies make bigger profits, clearly diversity boosts your business revenue". If that was really the case we would expect to find the same ting on national level i.e. more diverse nations being more richer. However the opposite is true. National diversity correlates with GDP per capita at -0.55.

There is also research showing the opposite - firm productivity decreasing as diversity rises. So who knows. There are some smaller meta-analysis finding negative relationship. Hulsheger and Anderson (2009) meta-analyzed 104 studies and found background diversity correlated at -.133 with innovation.

Williams and O’Reilly (1998) came to this conclusion in their review of the literature:

“There is substantial evidence from both laboratory and field studies conducted over the past four decades that variations in group composition can have important effects on group functioning. These studies show that increased diversity, especially in terms of age, tenure, and ethnicity, typically have negative effects on social integration, communication, and conflict.”

I wish there was bigger meta-analysis like with gender diversity but all available evidence so far suggests that diversity is bad rather than good.