Right. Let's compare 2e Mages and 3e Wizards.
Wizards get:
More spells, in total.
No limit to the number they can have in their spellbooks.
Guaranteed access to spells every time they level up.
An improved ability to learn new spells.
Easy access to the ability to craft magic items.
Faster xp progression at high level compared to other classes.
An increased ability to cast spells without interruption.
Increased probability that high-level enemies will not save against their spells.
Ways to make it harder for enemies to save against their spells.
Reduced drawbacks on many of the spells they cast.
Increased ability to get through spell resistance/magic resistance.
And I'm probably forgetting a few things.
Now, if you're going to claim that 2e Mages and 3e wizards are balanced, despite those increases in the power of the Wizard relative to the Mage, then you either need to show significant advantages that the Mage has, or equally significant advantages that other classes gained in the transition from 2e to 3e. Otherwise, the 'balance' that you claim is find in both editions would seem to be absent.
I'm not sure how 3.5E got better XP progression compared to other classes? I thought the XP progression was the same no matter the class?
Other classes gained quite a bit as well -
1) Fighters (and other fighting classes like Ranger, Paladin, etc) gained multiple attacks more rapidly and did significantly more damage because many feats were fighter-oriented. I can't remember how many times my 2E ranger would hit with his longsword for d8 damage, plus 1 for his 16 STR and then whatever plus he had for his current magic adjustment, so was hitting for 5-12 points of damage per hit at the end of the campaign when he has a +3 sword.
You no longer needed +2 or +3 weapons to even hit certain monsters - you just had to do more damage than their easily overcome DR.
2) Rogue sneak attack was significantly easier to use in 3.5E than in prior editions, where you needed to completely surprise an opponent to use it.
3) Clerics gained spells up to level 9, including the divine equivalent to Wish with Miracle. They only went up to level 7 in previous editions.
Clerics were also able to spontaneously cast Cure spells.
In going through a long running 3.5E campaign a few years back, I found both sides - the PCs and the bad guys - rarely failed their saves on those Save or Die/Save or Suck spells. In older editions, saving throws were far more dicey a proposition.
Monsters were also significantly tougher in 3.5E as well - more hit points and they'd get to apply STR bonuses to their damage rolls as well.
As I said above in this thread - I never felt that the party wizard, cleric or psion were overpowered in my 3.5E game when compared to the elf paladin, the human fighter, the dwarf fighter, the goliath barbarian or the human rogue in the party. Everybody had their shining moments in combats (i.e., the dwarf fighter hitting a balor with 5 attacks, including 2 crits, and doing a good 200 or more points of damage to it...or the goliath barbarian doing close to 130 points of damage with a single crit on a raging giant barbarian (she had a x4 crit modifier)).