S'mon said:
It was only with 3.0 that the rulebooks stated that with training any INT 10+ person could cast Wizard spells. Previously only rare exceptional individuals could become Wizards or _any_ character class - BXCMI D&D stated that 1% of the population could be Classed, which is the figure I use for PC-class in my current 3e campaign.
Take a look at your 2e PHB. Requirement for playing a wizard= Intelligence of 9. The fact that they went ahead and said that only 1% of the population is classed only proves how they couldn't be bothered to take the mechanics to any sort of logical conclusion. If an average intelligence PC could become a (poor) mage, then why can't anyone do it? The double standard of PC's to everyone else is at the heart of this discussion. Earlier editions didn't match the mechanics to the worlds they developed.
But let's go with this 1% figure for a second. The population of Germany in the 1300's was around 14 million people. That translates to 140 000 classed citizens. Figure a fairly even distribution of classes, which, considering the requirements for any given base class - Fighter, Cleric, Mage, Rogue - are pretty much identical, isn't a bad assumption, that gives us about 14 000 mages and 14 000 clerics. Roughly. Now, if the political power of that area could conscript/hire 10% of those, he's got 1400 mages and clerics to play with at any given time. Granted, most of those would be low level, 5th or less, but, then again, there are many low level spells which would make household magic very useful. Given a few decades, it wouldn't be difficult for a nation to build up a vast store of magical knowledge and whatnot. And this is the logical conclusion of a completely arbitrary 1% figure. Never mind that in earlier editions, only humans were zero level. All the demihumans were at least 1st level somethings.
Dnd has shaped modern fantasy just as much as its been shaped. Forcing common themes like magic to follow logical conclusions is a basic concept of campaign building. Fantasy and DnD is locked into a virtuous circle where concepts from one migrate to the other, get tossed around, changed, warped and made better then tossed back to the first.
Yes, there are examples of wizard protagonists prior to the last twenty years. That's true. However, VERY few people have read Vance, and two examples hardly make for an overwhelming change to the genre. It's been the last twenty years before you saw protagonist wizards in any numbers. While they did exist prior to about 1980, they were pretty few and far between and VASTLY outnumbered. Now, with Eddings, Wiess and Hickman, and Rowlings and Brooks and a host of other authors, wizards are frequently featured as the protagonist. This is a major change from earlier fantasty, say pre-1975.
/edit - taking things to their logical extention