One of the worst things Star Wars did was start making this same mistake. What was fun in the original movies was that the Empire was evil and bad. The Rebels were good. What was so fun was that Stormtroopers were inhuman bad guys you could kill without remorse, like shooting endless armies of battle droids. Luke Skywalker killed millions of Imperials by blowing up the Death Star, and it was awesome because he killed millions of bad guys.
Then they made the Empire maybe not so bad, humanizing Stormtroopers and making it all more realistic. Suddenly, Stormtroopers were people with families and such. Imperials might be people just fighting for legit peace and justice. They just aren't on the side of the Rebels. So who's bad or good anymore?
And just like that, it's not as much fun because now a single Stormtrooper death makes one wonder. Did I kill someone good who is just not on my side with my agenda? Where once I could run around in my backyard shooting stormtroopers for pretend and hacking them all down with a lightsaber and I'd feel like a superhero, now I must question the morality of my fantasy story. Was Luke actually the good guy or the bad guy? He murdered millions with one shot. These were people who had no say whatsoever about the decision of some murder hobo Moff. They were government employees just trying to work for a living.
Now, all my childhood heroes are villains because people want to make the fantasy story more realistic. Han, Luke, Leia... They're now all murderers butchering innocent people just because they want democracy instead of a monarchy.
That is what will eventually happen to D&D if people keep pursuing this "no race is really good or evil" line of thinking. This is not reality. That's why it's fun. Real war sucks. It's not fun. It's tragic and terrible. I don't want my fantasy adventure game to be too much like it or it will suck too.
We want to play a game where we don't have to feel bad for murdering countless monsters. I know a goblin is evil and baby monsters will eventually grow up to be adults who will eat people. No question. Kill the vermin. Wipe them out without remorse before they come and get you or others. I am a hero for exterminating them all because I know they're all bad without question. In fact, I can feel good about killing as many as possible because they are all bad. The world is a better place because I butchered the bad monsters and saved lots of people.
As for tieflings, yeah. Never evil. Neeshka from Neverwinter Nights 2 was the first tiefling I ever encountered. Never evil. Mischievous and devious, maybe, with a splash of moral ambiguity, yes, but not evil. There's quite a difference. Never were they man-eating vile murderous monsters as a race.
And finally, a one-off is fine. Kobalds like Deekin are rare. They certainly aren't the norm. So, although you might encounter a single decent goblin at some point, making for a fun and interesting companion, it isn't by far the norm, making killing other goblins or kobalds still good.
