FormerlyHemlock
Hero
For that matter, if you go to the nearest source of inspiration, Vance's Dying Earth, the magicians he imagined could cast only a handful of spells a day - even the greatest Vancian magicians ever alluded to in his works could manage to memorize at most 10 spells - something D&D wizards exceed while still in single-digit levels.
How familiar are you with the Dying Earth? Your description matches some Vancian wizards like Mazirian, who stuff spells into their brains by force of will, but neglects their mystic runes (reusable), technological cloning vats, and words of power (possibly reusable, possibly the same as spells). Other wizards such as Rhialto are far more powerful and don't rely on spell memorization at all AFAIK; they just bend reality to their will.
RE: "greatest wizards," I've never seen any source putting a limit on Pandelume's ability to memorize spells, nor Phandaal. We know Turjan and Mazirian were limited to less than six spells at a time, but they're not legendary. In fact, "legendary" Phandaal is famous not for the number of spells he could MEMORIZE but for the number he INVENTED. Knowledge is power for these wizards.
I don't think your assumptions about Dying Earth wizards are born out by the text.