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Pedantic Grognard
Well, from a brand perspective, it's a simple enough question. Are you trying to make a game consonant with the particular history, lore, and tradition of "Dungeons & Dragons", or are you going to do a "Reimagined Ideal New Fantasy RPG" and try to market it under the D&D name? Given the history of relative commercial success for various RPGs over the last five decades, if you as a brand manager want to avoid losing your job, you answer the first.(And yes - I know the obvious answer - if you do not like them in your game, do not use them. My question is not aimed at how to fix my world - it is aimed at what the D&D brand is doing overall).
In the very beginning, the game had six types of dragons (black, blue, golden, green, red, and white), plus hydras and wyverns. Those all then reappeared in AD&D, B/X, BECMI, AD&D 2nd, RC, D&D 3rd, D&D 3.5, and D&D 5th. So, that's your absolute minimum set of dragons and dragon-like creatures for the D&D brand.
If a DM is worried about there being too many types of dragons, you tell him he's free to exclude any he likes from his game world, but you keep them all for the brand.
If you then decide, as a brand manager, to add more dragons, the first four types to add are pretty much already decided for you -- the four types from the first D&D supplement (brass, bronze, copper, and silver) that then appeared in the core releases of AD&D, AD&D 2nd, D&D 3rd, D&D 3.5, and D&D 5th.
After that, you gain some flexibility on what you add. But if you try to break from the particular history, lore, and tradition of "Dungeons & Dragons", you'd just be begging to be "Pathfindered" out of a job.