The issue with this is that the class balance being based on a campaign basis as opposed to an encounter basis worked for AD&D but did not work for 3E.
In AD&D:
1. Magic Users did not get bonus spells for high Int, so they had less.
2. Most games didn't go far past level 10, if at all, and games that did tended to focus away from killing monsters.
3. AD&D played much faster at the table, and adventures tended to be written and run with more encounters between rest periods as a result of this. Combined with getting less spells due to the lack of bonus spells for stats, your memorized spells had to last longer.
4. Splatbook use was irregular, and a large amount of games didn't use many spells outside of the PHB.
5. Monsters tended to have low HP, and causing damage with spells was often the most effective thing you could do. Magic Missile and Fireball were staples of play at all levels.
6. Your ability to hit with Save or Die spells did not go up with your level or stats, but the ability of monsters to resist them did. Thus, at higher levels, these spells tended to hit infrequently.
In 3E:
1. Wizards get bonus spells for a higher Int, which gives them more spells than an AD&D ever had at a given level.
2. In 3E, higher level play was far more common and more focused on fighting enemies. The game breaking power of level 6+ spells had more of an effect on play.
3. Combat in particular and gameplay in general was far slower in 3E than it was in AD&D, and this caused adventures to be written and run with fewer encounters between rests, letting the Wizard's limited resources go farther.
4. Splatbook use was more common than sticking to just the PHB, and the wider selection of commonly used spells gave more bang for the buck than the average AD&D Magic User used to have.
5. Monsters tend to have 2-5 times the HP that they did in AD&D, but the damage dealt by spells was not scaled up. Dealing damage was most often a waste of time for the 3E Wizard.
6. Save or Die spells become more effective as you gain levels and stats, and can eclipse the enemies ability to resist them. Thus, they are more powerful and effective than they previously were.