It seems to me that a lot of the "problems" with high level wizards are mostly theoretical. Almost all of the arguments for why they're overpowered seem to just fall apart in the face of real circumstances. I played a wizard from 5th level up to epic levels, and only at the very end did things get ridiculous. The real breaking point is probably 17th level, when you get 9th level spells. Let's review a few of the arguments:
1. Anything you can do, I can do better.
Possibly true, but why would you want to? One example that's been thrown about is that Wizards make better scouts than Rogues, because they have Knock, Fly, and Invisibility. Yeah, Knock is better than Open Lock, but the Rogue can open as many locked doors as he wants without giving up any resources. Each single locked door/chest a wizard wants to unlock costs a 2nd level spell (which you'll have at max about 6 of). So on our scouting expedition, you're looking at expending a 3rd level spell (fly) and a ton of 2nd level spells (multiple Knocks, Invisibility, Silence) just to do what the Rogue can do all day long. And you still won't have anything that can see or hear enemies before they bash your head in, because the Rogue's Spot and Listen are way better than yours.
Yeah, a Cleric could become a better fighter than the fighter, but only for 2 combats a day and only if you give him 4 rounds to cast all his spells first, and only if the enemy doesn't see him glowing like a Christmas tree and cast a dispel magic on him.
2. Save or Dies make fighters expendable.
A single Save or Die has a very, very low chance of actually working against an enemy that matters. First of all, CR=level monsters with spell resistance have at least SR at least 10+lvl, so that means you're looking at a 45-50% chance to fail against spell resistance minimum. Yeah, you could take Spell Penetration and Greater Spell Penetration, but those cost 2 feats and only give you an extra 20% chance to succeed. Yeah, you could use Assay Spell Resistance, but those burn up 4th level spell slots mighty quickly.
And even if you beat spell resistance, you still need the enemy to fail their saving throw. Again, if you're attacking an enemy that matters, their saves are very, very good. You're only looking at a 10-20% chance that the enemy actually fails their save. So yeah, Save or Dies are great the 10% amount of times they work. The other 90% of the time you're just wasting resources. Save or Dies are really only good for when you're getting overwhelmed with medium threats that you have a decent (50%+ chance) of killing right away before they mob you. If you want to pull off a Save or Die on an important enemy, it'll take multiple rounds of stacking Enervations on them to give your SoD a chance to succeed.
3. I can just cast an "I Win" spell.
Pretty much every "I Win" spell is either very conditional, easily prevented/reversed, or has various other reasons why you wouldn't want to cast it all the time. Take Forcecage, for instance. It costs you 1,500gp a pop, which isn't peanuts even to a high-level character. It can be quickly bypassed by a lot of enemies. It doesn't do anything to actually kill the enemy. It's Close Range, which means you have to get in, for instance, breath weapon range of the dragon.
4. I'll never actually die because I have Contingency.
First of all, there's a million things out there that can kill you, and you only get one Contingency. And even if you're lucky enough to pick the one situation that you happen to run in to, it only saves your neck once. But secondly, every single defensive spell you're casting is taking the place of an offensive spell that you could be casting instead. So yeah, you could buff yourself to the gills, but it'll cost you a good chunk of your power to do so.
In Conclusion
In reality, I never had nearly enough spell slots to do everything that I wanted to do, or wished I could do, or needed to do. And you always had to pick exactly the spells you need for the day in advance, meaning that every wrong choice materially decreases your power. The Paladin in our group was extremely important even up to the last combat. We'd frequently get into a situation where we'd be fighting enemies who I could try to blast, but doing so would take 2-3 high level spells that I would really like to save, and I'm not even sure if it'll work, and the Paladin can chew them up in a few rounds anyways.
Secondly, you aren't acting in a vacuum. I spent a lot of my time Dispelling Magic because we were fighting something that kept throwing Blade Barriers in the way, or trapping us with Walls, or going Invisibile. At least half of time in combat was spent countering enemies' abilities instead of setting up my theoretically awesome combos.