It does very much water down things when anything can be anything, then nothing is really anything.
You....do realize that the fundamental nature of fiction, of any kind, IS that anything can be anything if you build it up to be so...?
That's the beautiful thing about our hobby. We can speak of whatever we wish to.
When your at the point of just pure randomness, then you have a mess of a game.
No, see, here you've erred. You have conflated "nothing has an inherent nature in fiction" with "so everything is 100% pure randomness."
The two are absolutely not the same. Remember, before Tolkien, "elf" meant things like Keeblers. Now it means gorgeous, elegant relics of a better time wasting away before they sail off to Heaven.
And.....you would sure disagree with a DM doing it, right? If some chicken eggs "just suddenly" grew into hostile tieflings and attacked your character. Would you be celebrating the DMs creativity?
I mean, it depends. Did they actually build up to that? Did we, for example, see some horrible mutated chickens lying dead beforehand? Did we see shattered, oversized eggs that smelled of smoke and sulfur? Did we have a chance to find out what was involved?
If the answer to at least most of those questions is "yes", then yeah, I might actually find that highly creative and interesting. It invites a great many questions. It also invites some humorous jabs based on Diogenes' famous criticism of Plato's Academy by throwing down a plucked chicken in the middle of the school and declaring, "Behold, a man!" because the Academy had defined "a man" to mean "a featherless biped".
The point would be a tiefling is a fairly specific thing. It is not "just whatever you want it to be on a whim", then it is nothing.
Nah. "Tiefling" has been many things over the years, and the specific
part of it that is essential to any given fan doesn't need to be the same as the specific part essential to a different fan. That's the nature of being a fan of some kind of fiction. Different people get different things out of it. It's good, and healthy, that D&D provide an experience where that can happen--it means it embraces a wide variety of players, who can then organize amongst themselves to find copacetic groups.
If a good DM really wanted a "new something", they could just make a new race to fit what is needed.
Or they can repurpose what exists. Happens all the time. WoW wanted bull-people, but didn't want all the baggage that tends to come with "minotaurs". So they invented tauren, which are...essentially minotaurs, with none of the cultural association of minotaurs (they aren't linked to mazes, they aren't monsters, they don't do anything horrendously awful, they're vegetarians, their motifs are Native American rather than Greek, etc.) Alternatively, sometimes you invert a usual association and keep everything else. Orcs and dwarves are subtypes of
elf in the Elder Scrolls universe--and the latter, Dwemer, haven't existed in the mortal world for thousands of years after they experimented with reality-altering magitech craziness and accidentally expunged (or possibly "ascended") their whole culture/species simultaneously.
Or, as mentioned above, what Tolkien did to elves (and dwarves, for that matter). He took something that existed, and reinvented it in such a compelling, exciting way, it's been the dominant narrative for almost ninety years (
The Hobbit was first published in 1937.)
If Tolkien were forbidden from re-inventing an existing fictional concept in a radical new way, most of fantasy fiction as we understand it today wouldn't exist. By your own arguments, Tolkien elves should never have been--and yet now they are inextricable from "fantasy" as we know it.