There is a difference between being a thiefling and look like a thiefling. I want to be able to reject outright the former without that creating too much fuzz. The latter I can be very happy to work with the player to acheive.
I don't think the difference is nearly as hard as you claim.
Given you don't care about appearance, I'll be focused purely on mechanics. In 5e, being a tiefling gives you three things, the middle of which comes as package deals associated with the three heritages, Abyssal, Cthonic, or Infernal). The first is 60' darkvision. Given this is something many species have, I doubt you care about that. The third thing is the
thaumaturgy cantrip, which again, I presume you have no problem with. So the whole focus is on the three-part packages.
I assume, for example, that you would not raise such objections with drow/dark elves as characters? Or just elves in general, really. Because guess what? Other than elves getting
even more features, they're functionally equivalent to tieflings! Elves get: 60' darkvision, the three-spells package deal selection (drow/wood/high), advantage on saves to avoid Charmed, one free skill proficiency from Insight/Perception/Survival,
and Trance.
Tieflings get 60' darkvision, the three-spells package deal selection, and Thaumaturgy.
That's it.
Is it really so hard to have--for example--a wildly divergent subspecies of elf, that was experimented upon to test the boundaries of magic, which resulted in them manifesting slightly different magical powers and
losing some of the benefits that come with being an elf? I could see such a group being ostracized and hated for their differences from regular elves (especially if they have the classical "bright red/purple/blue skin" appearance of tieflings!) despite not having
any connection whatsoever to anything fiendish since, as you have said, fiends don't exist. It'll be a different spin--racism
inflicted upon a people, rather than being an ancient "legacy of evil" or whatever--but when you don't care about the appearance...AND you accept the "the same but
objectively better" elf stuff...I just don't understand what the problem is.
If the player is okay with something that explicitly isn't, in any way, related to demons or devils or whatever, but still has the same appearance and the same abilities (perhaps reflavored?), I don't really get what the problem here is. Is it that the spells are mostly good...?
Who are in a position to not permit something?
GMs. I thought that was understood? Like you...literally used your own example of having permitted something mechanically broken that you shouldn't have, out of ignorance rather than apathy.
Someone who is apathetic about permitting something they believe to be problematic has kinda surrendered at least
some of their standing to argue that someone else caused a problem.
No, this one was actually quite hard to come up with. But there have been aluded to it in connection with nkn-trad games; like exploiting intent resolution to find rubies in drawers, or what would happen if someone corrupted by power started reading runes. In trad games hardly anyone are thinking like this.
So...people intentionally and actively trying to destroy the premise of the game they're playing...?
I just don't really see this being an actual problem. The people who floated such an idea have made abundantly clear they would never play in such a game and would never run such a game. I don't see what is being achieved here.
However I now remember I think I have one situation where I was close to having to apply this kind of line. A friend was having a d20 I thought seemed a bit suspicious, so I started recording all results from it for quite a few sessions. When presented with the collected data my player luckily volunteered to change die.
I think having this as an explicit formal power can be helpful for keeping certain types of players aproperiately focused on the game; in particular the more competative minded ones. (And these are not that uncommon in trad, which also reflects back on your comment on RPGs being cooperative)
I mean, I'm not against calling it out. Very,
very much the opposite. I just don't think it fits
in this place. That makes it sound like it is purely elective on the GM's part. It is not. Different GMs will apply personal judgment differently, but nobody will advocate approval of things they believe are outright harmful to the ability to
play a game at all.
This is a restriction required by the very act of playing a cooperative game. It should still be called out. But it should be put where it belongs: at the very heart of the activity, a "we outright HAVE to do this if we want to play" type thing.