Eh, I wouldn't take 'exception-based design' too far. It is intended to apply to the RULES, not to the GAME ELEMENTS, which are different. A race or a monster is a game element, it has what the rules say it has and nothing more or less. It may also have some defaults, but even those are situational and up to the judgement of the DM. One assumes if a monster is a humanoid with a speed of 6 that it walks, but that doesn't mean you would literally expect that a crippled legless human beggar will be walking and while he might have a stat block he could just be a statless 'bystander' NPC and clearly the DM will say "nope, that guy only crawls".
You can make that distinction. Or you might not. Folks approach the game in different ways.
So, likewise the DM is free to determine what an eladrin toddler can do, or for that matter a minor NPC eladrin. While there are many stat blocks and race rules for PCs of the eladrin race, and AFAIK they all have Fey Step its nowhere stated that ALL ELADRIN have Fey Step, or even most. Many of the rules that apply to adventurers and their ilk and to creatures with stat blocks may or may not apply to their entire race. In fact we can find cases where clearly a given ability does NOT extend to the whole race. So no 'exception' is needed if the DM wants to say "the shopkeeper can't teleport, in fact NO shopkeepers can" because there's no rule to be making the exception to. The only rule is "if its on the statblock/character sheet then the character can do it", or its a 'default', and there's never been a definitive list of default characteristics of creatures (nor would one make sense).
Well, look, tripping oozes doesn't make sense, and square fireballs don't make sense and HP-as-morale doesn't make sense but this doesn't stop them from being rules in the game. Heck, dragons don't make sense and wizards don't make sense and the Feywild doesn't make sense and healing magic doesn't make sense and...
What one chooses to "make sense" in their world can be different from table to table. Is Eladrin teleportation something every eladrin can do from birth or is it only a facet of training elite eladrin or something else you want to make up as some excuse for why this doesn't affect the world (and if that's the case, why isn't there an option for a PC to swap it out?)? Choices like this inform the world you make in interesting ways, and there's no one right and true answer that should be the case at every table.
DMs are clearly entitled to their opinions and ideas, but it seems pretty inflexible for a DM to say "I have this terrible issue with this one aspect of a setting which aspect I in fact extrapolated as being the case and could change trivially at the drop of a hat." Its not so much that I feel like I have an issue with said DM as much as I just can't help rolling my eyes when these things come up.
So, first of all, consumers pay for the game and if the game they pay for doesn't deliver the experience they want, then that game has failed to deliver value to them, and they get to criticize it. At the very least, the design team has failed to properly telegraph what this element is for and how to use it to that player, and it's a chance to learn where the failures happened and how to improve the design in the future for how it is actually going to be used rather than how the team intends it to be used. Experiences aren't right or wrong, they are experiences, and play experiences are worth analyzing if you want to improve your game. And bad play experiences are especially valuable for that.
Second, this is only inflexible if your end goal is to have a game with teleporting eladrin in it. Not everyone is invested in that goal. Not everyone CARES about having teleporting eladrin in their game. If your end goal is to have a fun game experience, you may not need teleporting eladrin, so it's actually quite flexible to say, "Screw that, no Eladrin in my games." That's adapting the rules to your needs as a DM or player -- you don't like it, so they're out. No one HAS to play a D&D game with blink elves if they don't want to. They're not important. They can be useful, but lots of things can be useful, so that's not really very special.
My point is that it's
legitimate, as legitimate as any other complaint about D&D has ever been over the lifespan of the whole friggin' game. And people can disagree about legitimate complaints -- if they share them, if their experiences are different. Dismissing it as edition warring or rolling your eyes or saying that it's an "incorrect application of the rules" is an attempt to say "Your experiences don't matter. They're not legitimate experiences. You're making it up to push an agenda, or you're doing it wrong." And that's just not the case.
Some folks imagine eladrin teleportation would be huge for a world, and don't like it because of that, and that's OK. They don't have to believe otherwise.