There are four scenarios worth considering, representing the ends of two "dials" on magic - how recent it is, and how prevalent/practical it is:
- Magic has always been around, but it's incredible rare/difficult.
- Magic has always been around, and it's very common/easy.
- Magic has only recently been discovered, and it's incredibly rare/difficult.
- Magic has only recently been discovered, and it's very common/easy.
There are any number of possible explanations, but the point remains - a small amount of magic, even if around for a great deal of time, would not have any significant effect.
Scenario 2 would be wildy, radically different from our world. As different as the world of a modern Briton is from an agrarian Brit living under the Norman yoke in 1088. Positing a world where anyone can do useful magic with the equivalent of, say, a six-week nightschool course would result in a world where everyone who's anyone retains one or more potent magic-users. Magic is used in industry, warfare, trade, diplomacy... Much as computers have transformed every facet of our current world, plentiful cheap magic would transform the ancient world, and therefore transform every subsequent age beyond recognition.
Instantaneous communication, teleportation, magical lie detection, purification of elements, transubstantiation of elements, battlefield artillery, mass production of wondrous objects... The possibilities are limitless, and it is difficult to make any kind of prediction about what such a world would be like. It would depend greatly on the exact types of magic that are feasible.
But to argue that plentiful, cheap magic, available over the long term, would not cause at least the degree of transformation that computers and industrialization did is insupportable.
Scenario 4 would be a world like our own transitioning into a different kind of world, like England at the beginning of the Industrial Revolution. Forward-thinking businessmen and princes would be making the most of the new-found magic in industry, commerce, and war. Most people would still be living pre-revolution lives.
Perhaps centuries of alchemical and astrological work were converging on a single solution to the problem of practical magic, and near-simultaneous discoveries were made in several places. Perhaps some planetary convergence opened rifts to another world. Perhaps powerful beings have returned to this realm, bringing magical potential with them.
None of this is Earth-shattering. I wouldn't think this is particularly controversial even (although I might be surprised). I don't think Alexis and I disagreed so much in our conclusions as our premises - if I understand correctly, he's thinking of a Scenario 1 world, and I'm looking at all the possibilities.
That said, we play a game. If you want to play a game where powerful, plentiful magic has always existed, but the borders of Europe are the same in 1088, the feudal system is in place in Norman England, and in every way the world mimics our own (but there's powerful, cheap magic everywhere), all power to you. It's a game. Do what works for your game!
But don't try and tell me that's a plausible historical scenario. It's not.