Second, it's far easier to destroy/remove, than add/create. It takes hours to build a good card house, but 1 second and a slight bump of a pinky to bring it down.
Only if the analogy is sound: something that has no connections outside itself, other than the physical surface upon which it stands. I don't think that works for species or cultures in D&D. It's too interconnected at the local level, and there's too much background context at the gaming-culture (IRL) level.
If you're putting in a ton of work either way, you're putting in a ton of work whether or not a specific race is present. Choosing to build a world that lacks elves is going to imply some extra work on your part, for example, because so much of the fiction and (IRL) culture surrounding D&D includes things like elves and halflings--you have to do more work
yourself, as it were.
Cultures generally aren't self-contained, but rather constrained by available transportation and the like. Hence why I've (repeatedly) referenced the whole "beyond the horizon is your friend" thing. In a medieval world, even places that are physically connected (the way, say, China is physically connected to the Iberian Peninsula) can still be sufficiently "separated" that they are curious unknowns from which many strange and wondrous things could arise without, strictly speaking, being a Problem per se. Couple that with the possibility of
actually developed cultures living beyond an ocean or other much-more-difficult barrier to travel, and you get a ripe opportunity for introducing things that don't need tons of foreknown and pre-established context, but can still be received with curiosity, wonder, or greed rather than hate and fear.
E.g. maybe there's a (relatively) advanced dragonborn nation on the opposite side of the Nebow* Ocean, and now it wants to make trade overtures; that could be a great reason for a diplomatic envoy character, who is making those initial overtures of peace and trying to assuage concerns. Or if you have any kind of other worlds, they could come from one of those, and
for them this strange world populated by humans might be as Weird and Magical as the Feywild is to humans. Which could be an incredibly cool story!
Again, you aren't
required to do any of these things, but keeping an open mind makes so many interesting paths available to you and your group. That's all. And keeping an open mind
does not mean needing to suddenly do a ton of extra work yourself. Working with your players, intentionally leaving things lightly-sketched and allowing spontaneous ideas to fill in the blanks, or stealing ideas from the players as they speculate, all great ways to lighten your burdens while still producing quality work. Or maybe you just take it a week at a time; the players can't
learn every single detail about a new culture in a single session. Etc.
*Nearby Enormous Body Of Water, naturally
