I dislike the retcon because it detracts from D&D's lore without adding anything. I'm flabbergasted how something like hoard thieves (or whatever those parasites are called) could take priority over things like steel & song dragons. The "some metallic dragons prefer to live in humanoid form" is a one-liner suitable on a personality trait table – not even worth a sidebar.
Retcons are fine (IMHO) when they add something to the game.
For instance, they could have said "Steel dragons is a catch-all for spellcasting metallic dragons who rebelled against Bahamut's rule and the Council of Wyrm's mandate that dragon not interfere with mortal politics, preferring to live in humanoid form. The term harkens to the Song of Steel and Talon describing how Bahamut formed the Council of Wyrms. Amiable meddlers, these dragons pose as sages, scholars, and mages, and seek to shape the course of human kingdoms."
That kind of retcon would have been more palatable to me, and changed my opinion to be neutral to mildly positive. Yeah, it would seem like an unnecessary change but the lore would be so interesting that the positives would outweigh the negatives.
This isn't just theory craft.
I used a steel dragon in my old AD&D Planescape campaign who masqueraded as a gnomish antiquities dealer, showing up in various cities to barter and swap lore with the PCs. He ended up publishing one PC's book on languages. There was always some hint at his draconic nature, like steel-grey eyes, a scale pattern to his doublet, a scarf seeming to be made of steel mesh, oddities so ancient they far eclipsed the gnomish lifespan, instantly recognizing the various types of steel (cold, Baatorian green, Abyssal bloodiron, etc), a coiled dragon ring, off-handedly referencing some event with firsthand intimacy that occurred ages ago, etc.
Part of the story arc was the PCs were trying to shift a town that was slipping across the Outlands (and towards the Abyss) back into Arcadia. One of the sub-quests to achieve this goal involved enticing lawful merchants to resume trading activity with the town. The PCs decided to convince their "gnomish" friend to do business with the town, and that's when his backstory was revealed about having a disagreement with his draconic brethren that led to him traveling the planes, studying humanoid cultures in hopes of creating a utopia to prove his jaded brethren wrong. The PCs ended up debating philosophy with two elder dragons to convince them how this town would be one form of utopia once it transitioned back to Arcadia. It was a lot of fun playing with the "we know he's a dragon, but no one is coming out and saying it." And the dragon sibling "politics" was a hoot.
