Not who you were responding to and I honestly don't feel strongly about the subject. However, I have to say that is not my interpretation of the reaction. First, from what I recall the characters are actually pretty different. Tiamat is basically can't believe it's not hydra Godzilla while Takhisis is a scheming overlord. Sure, they are both shaped the same. But the actual character portrayal is significantly different from my experience. Saying that's just different "history" is like saying two twins who look the same but lead completely different lives are really the same person, the different "history" isn't important. But it's entirely possible that is just due to my limited knowledge.
No the thing that I think is really wrong about this is the specific sentiment quoted above. It seems less to me that people are upset because Takhisis ABSOLUTELY CAN'T "just" be Tiamat. She ABSOLUTELY MUST be an Original Character Do Not Steal. Instead they are annoyed that WOTC is saying Takhisis ABSOLUTELY MUST "be" Tiamat. She ABSOLUTELY CAN'T be an Original Character, There Can Be Only One. There is a world of difference between insisting that one thing must be, and disagreeing with someone else insisting one thing must be. The fact those are very different takes doesn't change whether it's fans, creators, corporations, or acid fueled talking spider trees.
Obviously, multiple people have different viewpoints, but that is mostly the vibe I've gotten. Disclaimer that I haven't done in depth research on it. But I have noticed in this thread alone a trend to exaggerate and mock people who are basically just saying "this is unnecessary, stop trying to enforce an ill thought out all encompassing canon on everything, it works better leaving all that alone". The Original Character Do Not steal thing above is an example. It's very clearly a reference to overdramatic fanfiction divas. People like to say don't yuck somebody else's yum. And it's a very good point. But it's also bad to try to force your yum on somebody else's who thinks it's yucky.
I mean, that's all fair.
But it's more than a little frustrating to have my seemingly-justified comparison (looking at their physical appearance, overall personality, behavior, interests, spheres of divine influence, etc.) dismissed as apparently "quaint." I get that they've gotten up to different things, but I haven't seen anything that suggests that Tiamat is "just" Fantasy Godzilla. Indeed, I've always thought of her more as an incredibly dangerous antagonist, somewhere between the characters Hexadecimal and Megabyte: she
can verge on the shrieking insanity of Hex, but generally acts more like the incredibly dangerous schemer.
I will admit, part of the reason I feel this way is 4e. In 4e, Bahamut and Tiamat were full-on proper deities (equivalent to Moradin, Pelor, Sehanine, Torog, etc.), and the background history regarding Arkhosia (one of my favorite bits of D&D lore) specifically reflects her power and her
subversiveness. Despite the empire being
explicitly dedicated to Bahamut, with special reverence also paid to the trio of (notably Unaligned!) supporting deities Erathis (civilization and law), Ioun (knowledge and skills), and Kord (storms and strength), and Tiamat worship being outright illegal, her temples continued to exist as subversive cults across the Empire, and in its final century or two they were
rampant, sometimes even to the point that worship occurred
almost openly.
That's not the kind of thing that Godzilla does. That's the kind of thing a conniving, and more importantly
patient, adversary does when faced with overwhelming force. Don't fling yourself at the enemy's shield.
Wait until their arm gets tired.
Edit: Further...it's not like I'm being demeaning here. I literally pointed out how this actually demonstrates that Dragonlance, for all the crap people fling at it, has
permanently changed the lore of D&D as a whole. That recognizing the deep (but not
totally all-encompassing) connection between Tiamat and Takhisis is, in a very meaningful sense, showing how Dragonlance has asserted its own canon over what came before. So it's frustrating to be simultaneously told not to give flippant responses that ignore important context
in a way that is kind of flippant and ignores important context.