This is no more true than it is to say there should only be one race today because whites and blacks and Asians etc would have race wars and exterminated each other.
So basically, I’m saying I disagree with your logic
I terms of what D&D calls "races" there is only one. The fact that you think there are massive irreconcilable differences between whites, blacks and asians demonstrates a massive misunderstanding on your part.
There is less diversity in all of humanity, yes-- that means between what you think the extremes of "blacks, white and asians" is-- than there is within a single clan of chimpanzees. Oh, and of that diversity that exists? Yeah, 70% of it exists within the people you group together under the label of "black".
There were other humans on planet earth-- Neanderthals, Erectus, and descendants of Australopithecus that ranged from giant and meaty 8' tall giants to 3' tall "hobbits" with features that spanned the range from chimpanzee to what we would currently call human.
Everything that diverted much from the common human norm is completely extinct. Apparently "whites" carry a notably high percentage of neanderthal genes, but all in all... the genetic "eve" and "adam" date back only a few 10,000s of years (the genetic eve is actually much more recent than the genetic adam, for whatever little that means) of the 100,000 years that the modern strain of human has been around and the 500,000 years that humans of any shape and size have been around.
So... "sorry" to say-- but his logic is pretty damn sound. If given the opportunity (an opportunity that whites, blacks and asians never had, mind you) humanity would in fact entirely exterminate absolutely all similar creatures that could be defined as "different" that existed within their same region.
Moreover, the very fact that the genetic "groups" you list (which, again, are faulty due to the fact that the vast majority of diversity within humanity exists within one of the things you decide to group as a singular thing) is evidence of humans violently eliminating those who were in the minority.
Another issue I have with this take is Human races are sort of coded as "white" while monster races have been coded as "non-white" and I hate that unintentional racism. I mean if you have magic what is so hard about a goblin wanting to be a courageous Paladin?
What would remotely make you think it was unintentional?
The first few editions of the game admitted their introduction to roleplaying was "cowboys and Injins" in which... yes, the Cowboys were always all white (regardless of the fact that is historically wildly incorrect, it is what Hollywood portrayed) and the Injins were non-whites who were therefore evil and therefore deserving of extermination-- thus the creation of the U.S. was righteous and holy and nothing one should ever question the morality of.
Why wouldn't a game setting created by people with such a mentality not strongly and explicitly convey such ideas and concepts as they were brought up on and so very ingrained in their ideas of what roleplaying was all about?
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Anyway, I resort to what I said in my first post. The OP has little business running D&D period. This all traces back to a powermad DM who is incapable of comprehending the simple fact that this is a cooperative game and all people involved need to be enjoying it or there is no point in running a game at all.
If one wants to decide who the PCs are and limit their options in the world to an unreasonable degree and kill them off if they do anything the DM doesn't expect-- that DM needs to not be DMing and instead needs to just go and write a novel, because they are out-and-out far to anti-social to be engaging in a game like this.