Interesting line of thought. So in our hypothetical no-spell-casters campaign, I can reskin my wizard character as a martial and use my spells mechanically as written?
Here's my simple question: WTF are you doing using (non-4e) D&D for a no-casters campaign. About 40% of every single non-4e PHB is spells. About two thirds of classes are casters.
On the other hand I
have played a bard as a pure charlatan who didn't actually have any spells, but was capable of coincidental trickery. This isn't "I reskinned fireball" but "I was very careful and detailed in the spells I picked".
I do not think that this is a question of a map filled to the maximum but rather a question of continuity. Refusing to add more races can be justified in many ways, the gods are not the least of them. Especially in a world with only one pantheon. The gods are perfectly able and willing to prevent planar travel for a large size population or a single person if it suits their fancy.
If you can use gods as an excuse to
not allow races then you can certainly use them as an excuse
to allow races. Which means that the continuity argument vanishes in a puff of smoke.
I suspect that a very large part of this problem is that all of the "official" settings for the past twenty years have been fantasy kitchen sinks, with even the more Arthurian/Tolkiensque Dragonlance and the Gothic Ravenloft being outsourced to third-parties. As much as I bitch about Dragonborn and Tieflings in Dark Sun-- and the sidebar about Divine classes-- it seems like there's a lot less pressure to turn a strongly thematic setting into a kitchen sink than there is to make the existing kitchen sinks broader.
But... my problem here is the same as my problem with the Radiant Triangle in Spelljammer: when a setting already has so little identity, those minor exclusions are the only thing that differentiates one from another, the only things those settings have left. When you drink out of the kitchen sink, the only flavor you taste is dishwater; mixing the dishwater out of a dozen different kitchen sinks isn't going to make it taste better.
Of course, it's also those little exclusions and those little incompatibilities that made AD&D's classic campaign settings exclusive, and led to their ultimate commercial failure. I can understand WotC's desire to avoid making those same mistakes all over again... but that doesn't mean I have to like it.
And my problem with this argument is that D&D races are in general not that flavourful. They are light stereotypes that provide a little background colour - and most fantasy worlds are too homogenous anyway because there are only a few minds working on them. Humans of any culture are far more diverse than just about any fantasy setting even with the range of fantasy races.
The only race I can think of that has a
serious impact on the themes of the setting for good or ill is the Warforged of Eberron. The Last War lasted about 100 years and ended only a couple of years ago. But Warforged are new; the oldest playable warforged types are 1d12 years old for the simplest types (fighters and barbarians) to 1d4 years old for the most complex (wizards, artificers). Also from memory there was a ban on new warforged two years ago -so roll a 1 and you're illegal.
Does mass production of warforged in little more than the past decade say a lot about Eberron on its own?
Definitely. And it does so in ways that the normal kitchen sink melange of elves (of all types), dwarves, orcs, gnomes, halflings, etc. don't.
But does this mean that I can't play a warforged in another setting? Of course not. A warforged is essentially an android made via magic. I've played a warforged made centuries ago by a mad wizard and trapped for most of that time. Did it disrupt the setting much? No. Mad wizards are a thing and adventurers are weird. There
may have been other warforged in the setting but I'm not aware of any.
And likewise small communities and far away communities disrupt very few settings and add a tiny amount of spice. You don't get a stronger setting by excluding things - you get one by picking things to focus on.