I'm going to take hit on this, but...this goes back to what I've said a million times before - balance is BS!. Balance is neither a good thing nor a requirement for D&D. 4e's everyone is the same is a sticking point for me. 3e was bad enough. (I've played both, ran campaigns in both and am not disparaging any edition, per se)
Unless you want to not be a wizard, at which point balance matters quite a bit more.
This idea that a wizard is the most powerful thing at higher levels is what is supposed to happen. But since DMs try not to kill PCs anymore, the idea that you actually have to make it to higher levels get lost in the sauce.
Sure a 7th level caster could do some pretty cool stuff, but they had to get there first and a kobold with a lucky crit could end the career of a wizard as easily as a lowly snake and a failed poison save. Does nobody remember the phrase, "wizards are squishy"? Or how about the converse, "Hey, meat shield, protect the caster."
Because this isn't balance.
Hey guess what fighter? Level 1? That's it. That's as good as it gets. Really! Because the game is "balanced" over levels. So enjoy level one, because every level afterwards you get closer to being obsolete.
Incidentally, your job is to be personal bodyguard to the wizard. Yeah, you get to be a hireling! Doesn't that sound fun?
Know what it doesn't sound like? Any story, ever.
Read mythology. No, I'm serious, go read a few myths, read some fiction. Go read Greek mythology or Japanese mythology, read up on the Three Kingdoms and on Roland and on Gilgamesh. Now point out the number of "meatshields" who do nothing but protect the wizard. And point out the number of godly superpower wizards that are the protagonist.
I'll wait.
Now point out how many stories are "And this this rediculously awesome warrior guy did something way over the top and basically impossible and it was really rad."
I think the comparisons support me far more then they do you.
In fantasy genres balance should be a dirty word unless you are talking about thief skills.
Only if you want the game to be Wizards and Wizards: A Wizardplaying Game.