Does this thread assume that it is equally as easy for anyone to become a Spellcaster level 20 with access to 9th level spells as it is for anyone else?
I've been designing a
Transmutation War Weaver complete with feats and alternate class features up the ying yang with the assumption that this character was trained by a Transmutation Specialist Wizard. He has 3 prohibited schools and some nifty feats and things because from day 1 he was taught that way. His teacher assumed Necromancy, Evocation and Enchantment were useless and so didn't bother to teach him anything from those schools.
If everyone could become a Wizard, than I'd foresee serious specializing happening. If one Wizard spends a lifetime refining Magic, only to teach those refinements to the apprentices of the next generation, Etc, expect Metamagic combos, "broken" PrCs and powerful spell usage to become more common.
Unless some powerful somebody(ies) regulates all this, see my earlier post.
If that powerful magic is Not accessible to all, and it's more a "everyone is a little magical", then Eberron's Dragonmarks and the Dragonmark feats might be a good model for this.