Purity Has Never Been Good
So, as of late, I've been listening to a lot of history in the form of Blue from Overly Sarcastic Productions and I, and hopefully many others, have noticed that the most prosperous societies tended to be the ones that accepted everyone equally regardless of how they were different from the natives, and that these societies fell apart once someone came to power with the idea that they should undo all that and make their nation only for the people they deemed as their people.
Egypt, Persia, Rome, the Ottomans... These are some of them over the past several thousand years.
Basically, purity marks the beginning of the downfall of empires. And it's not just in societies. Alloys tend to be much stronger and more useful than pure metals (though not always, because slag could ruin your iron pretty handily).
Being dogmatically monomaniacal, or, to use smaller words, being crazy about purity, has historically proven to cause a lot of damage.
Something I think I read somewhere, but can't find so I might have made it up, is "as soon as it becomes us versus them, everybody loses."
I think there was also one about how a people referred to themselves as "The People" and when they encountered another culture they called them "The Other," but things would have gone much better they instead called them "Also The People."
So, basically, everyone being different is much more advantageous to humanity than everyone being the same.
Intolerance is destructive, so destroy intolerance first.