Well, dang, this thread got very fixated on grave digging very fast. The internet is a hell of a drug.
The answers depend a lot on your starting assumptions. If you're taking the approach recommended by
@Oofta to start with the fantastical world you want and work backwards to things that must be true about magic and the in-world expression of the game rules, you could use a historical model without it being terribly thrown off. And that's a good way to go.
There are many ways of deciding what level of magic there is in the world.
To me, the more interesting thought exercise is to start with the assumptions and then try to figure out logically what kind of world there would be. It's an impossible task, but can lead you to weird places... which is why it's fun.
I tried working through this exercise with the 5e core books a little while ago and found myself going in a unexpectedly anthropological direction. These are my assumptions, which kind of followed from one to the next as I thought about it:
- Magic comes from gods and fundamental principles of reality that predate civilization. Magic would have shaped life and civilization from its infancy, which means that it doesn’t make a lot of sense to start with a level of technology (i.e. renaissance, medieval, etc.) and then add magic; magic needs to be baked in from hunting and gathering forward.
- This means that magic has been around long enough to cause civilizational selection pressure. So, while there may at one time have been groups which feared magic or persecuted magic users, those folks aren’t around anymore. They were all outcompeted thousands (millions?) of years ago by groups which made effective use of magic.
- It stands to reason that magic would be extremely prevalent—as prevalent as it is possible for mortals to make it—since it gives those who have it or have more of it strong advantages over those who don’t.
- So why wouldn’t everyone be a magic user? Possibly some species are (high elves, drow, forest gnomes, and tieflings just in the PHB), but I’m going to make further assumptions based on generic fantasy conventions and the text of the 5e core books, that there are some substantial practical limitations which prevent it:
- I doubt that a god or similar entity is willing to delegate its powers to more than single digit percentages of any mortal polity. The PHB notes for example that “Not every acolyte or officiant at a temple or shrine is a cleric” and “the gods don't grant this power to everyone who seeks it” (both PHB 56). Divine beings are, in general, jealous of sharing their power.
- Similarly, externally powered casters probably can't serve the competition/fitness priorities of the societies they are part of, since their supernatural benefactors will impose other priorities. So, for example, if druids are “concerned with the delicate ecological balance that sustains plant and animal life, and the need for civilized folk to live in harmony with nature” (PHB 65), you can't really have an agrarian empire that relies on them to cast plant growth to sustain its large population.
- Sorcerous powers that are inborn/genetically inherited are pretty limiting on magic as a technology. Excepting a massive state run breeding program far exceeding, say, that of Dune's Bene Gesserit, genetically inherited magical ability would be hard to scale.
- So then, everyone is a wizard?
- No, but lots are, maybe most are.
- If people can learn to be wizards through study and practice, like the way a 21st century person can learn to synthesize dangerous chemicals, build electronic devices, carry out surgery, or play concert violin--but these skills are ALSO the cutting edge of military, economic, and technological advancement--then society would optimize to train as many of these people as possible.
- Potentially, this group includes bards, but I'm not sure if their abilities are partially inborn or fully learned.
- Wizard's and bards wouldn't be the equivalent of 21st century engineers and lawyers, or the equivalent of college graduates, they'd be the equivalent of 21st century people who can read, write, and use a computer--if you can't do those things, you are at a huge disadvantage compared to the very large group of people who can.
- And given that this paradigm, potentially, arises very early in sapient social development, there are lots of things that we see as inevitable, that a magic-having society might never do.
- Lets say that early tribal and clan based societies learn that they can survive indefinitely by having a druid cast goodberry multiple times a day for them. 1st level druids feed 20, 5th level druids feed 90, 10th level druids feed 150.
- Druids don't like to do this very much--but one day one clan anywhere in the world realizes that they can reliably train college of lore bards to do the same thing.
- This society can scale up to billions of people without ever developing agriculture (apart from cultivating sprigs of mistletoe, but technically that's arboriculture).
- Every other society would copy it ASAP or be destroyed.
- And without agriculture, nothing from any premodern society makes sense.
- There would be no wars of conquest--since it's people and not land that creates value (though I'm sure there would still be wars of some kind).
- Technological development or progress would be vanishingly rare--since technological innovations would be less efficient than magical solutions that are already available.
- Families would be small, like they are now--since, unlike in premodern societies, children aren't a cheap source of labor on the farm.
- Gender roles would be egalitarian--since neither gender has physical advantages in casting spells.
- And all kinds of other weirdness.
Part of the issue is that I'm not sure that we know what society would look like if there was magic. But ... and this is a big but ... why do we assume that magic is attainable by everyone? Why assume that magic is generally beneficial? For every wizard that wants to better society, there's a wizard who just wants to watch the world burn.
Then there may just be some natural aptitude required to use magic, particularly arcane magic. You can say that natural selection would encourage arcane magic aptitude, but that's kind of like saying natural selection would encourage intelligence, and it has. But it depends on how you look at it. Compared to the great apes we're geniuses. But high level intelligence comes at a cost, and there's a pretty vast spectrum of innate capabilities on several spectrums. In addition, advancement of technology (or magic) is not a given. Our ancestors used bone or stone tools for millennia, it was not because they were unintelligent. It's just that multiple things had to come together for them to advance technology wise.
In my own campaign world, I do assume that humans have evolved with magic. It just doesn't have big flashy implications like everyone having PC class levels. Magic is ubiquitous, but not flashy.
When it comes to divine magic, you're assuming anyone that wants to cast divine magic can. I don't agree. Magic has to come from somewhere, power never comes for free. In my campaign the power of the gods that can affect mortals is really just power that comes from mortals in the form of prayer and devotion. Each prayer is like a raindrop, get enough raindrops and you get a lake but lakes can be drained more quickly than they are filled.
Druids get their power from nature and living things, constantly feeding everyone on goodberries could be seen as a massive power drain form nature with unacceptable side effects. Just as important, food and eating is a very important part of our life and culture. More on that below. So druids can bless fields and double the output of crops which is a good thing because it puts less pressure on the wildlands. But the crops still need to be planted, harvested and tended. On a side note, druids also provide free birth control in my campaign in large part to control the spread of civilization. Since child mortality is much lower (again, magic) than it was historically, the population is relatively stable.
I think dragons and other airborne threats would lead large societies to build their settlements mostly underground. With sufficient magical food and light they wouldn't need to go outside very much.
Many cities have lived with air assaults for a century now. We don't build cities underground, although we do have bunkers. Instead we have antiaircraft defenses. Make it dangerous for dragons to attack cities and they won't attack.
I think these takes are mistaken. If casters are extremely valuable, society is going to make the maximum amount of them it can--and wizards are probably the least bottlenecked class of casters a society can produce.
If becoming a wizard is like becoming a physicist then it doesn't matter how many people would
like to be a wizard, only a few can actually attain it.
Except that the druids might be philosophically opposed to this arrangement.
I agree that this would, potentially, be one of the civilizational game-changer spells, along with continual flame (permanent indoor light), lesser restoration (end disease), and wall of stone (infinite construction materials).
You're suggesting a very radical world, almost unrecognizable. That's fine, but it can also cause other issues of relatability. People need to feel comfortable with your world and feel like it's "real". Change too many things, make it too foreign, to distinct, from our reality and people will be less able to relate to that world. If people are happy eating goodberries for their entire lives that dramatically reduces the need for taverns and the like but it also many of our cultural touchpoints. You wouldn't have feast days, you wouldn't have that family gathering around the dinner table. Heck, you wouldn't have a dinner table.
If you want to look at a world where magic is ubiquitous, I'd suggest also looking at Eberron where magic in many ways is indistinguishable from technology. But go too crazy with magic? It might work with the right group, but it's likely to be more interesting as a thought experiment than an actual campaign world.