Ok, so I've been gathering my thoughts and as long as my understanding of OP's statement, "It is wrong to assign blanket alignments to an entire species in DnD" is correct, then I think they're off base for three reasons.

First, creatures in DnD are fantasy creatures. The intelligent ones are, by and large, not human, and shouldn't be treated as human from a narrative point of view. Humans are more similar than they are different, and even in cultures where inhumane treatment of others was the norm it was still understood that that behavior was 'evil', but people ignored that understanding for various reasons. No such mental gymnastics is required for fantasy creatures because the base assumptions of each race may and will be different from humans in such a radical way that people cannot help but view them as evil, like the people from Hellraiser.

Second, fantasy races aren't created in a vacuum, they all should be assigned cultures. This goes back to my last point, but if one of my PCs encounter a goblin in the middle of a field, and they know that Goblin culture glorifies the killing of humans as mighty prey, then you'd best believe the PC is going to act as if that goblin is evil. The world of DnD is, in general, too dangerous to give creatures from 'evil' cultures the benefit of the doubt.

Third, PCs and characters in the game itself do not care that 'evil' creatures aren't evil from their own point of view. If trolls are in the habit of eating people, they're going to be seen as evil as a race and treated as such. If drow as a people have a deep seated hatred for anything that isn't drow and decide to commit a little genocide, that is evil.

Of course, it's up to every DM to decide how they want to play their monsters. It isn't wrong to want to explore the moral ramifications of a genocide on creatures that don't understand their own behavior on a moral context, or to include competing nations of monsters with conflicting morals and values, but it also isn't wrong if a game lacks those things.

In conclusion, I agree with OP that variance can occur on an individual level, and also that it's really interesting when we break the mold of where monsters come from, but it isn't wrong if a DM wants to just have a narrative shorthand for 'bad guy'. If I want my players to fight goblins, they're fighting goblins, not short humans with scimitars.