A diversified population is bad for the same reasons that walking into a pharmacy and sampling one of everything is bad. Introducing every known chemical into a water supply is a recipe for disaster even if many of them are beneficial. It only takes one pathogen.
And that's the side of the argument the delusional people who think the world is a giant Coke commercial where we're all just a few open borders away from holding hands and singing "I'd like to teach the world to sing..." in a field of daisies refuse to accept. Inclusion is fine, diversity works, but it must be approached with a little common sense. Diversity as a goal is fine, doing it blindly expecting it all to work itself out is a stupid idea.
That's a very weak analogy though. Your analogy skews the argument, as why the hell would anyone ever sample every drug? Is the sampler even actually ill in this analogy? Would it work out any better if they took a million doses of the same drug? It doesn't really make sense as a scenario. Why not say a homogeneous population is bad for the same reason that stocking a pharmacy only with one kind of drug is bad - as you'll be stuck if you develop an illness that drug doesn't treat?
Diversity is not synonymous with open borders in any case. Two different discussions. Like I said already, I do think a very diverse population risks giving more power to those at the top, because its very hard to organise across language and cultural differences, but its a trade off.
