I don't agree at all that imbalance is essential to the "D&D-ness" of D&D. Sure, every edition of D&D (other than 4E, which you fail to acknowledge) has has a severe imbalance between Fighters and Wizard. This is not precedent for keeping that imbalance, it is evidence for the claim that they were flawed games that need improvement. That imbalance is a problem, not something to be happy about.
I choose not to acknowledge 4e because I do not have enough experience with it to know whether it destroyed the magic/mundane distinction enough to have "fixed" the balance issues. I do, however, posit that it must either be balanced or be D&D, it can't be both. I also believe that the basic issue of high-level casters pervades through every version of D&D, as well as its spin-offs, including PF, TB, FC, and various retroclones. At what point would this idea become "precedent" rather than "mistake". How many successful and enjoyable games have to be played with this melange of classic D&D rulesets before this paradigm becomes valid?
It is fine to want magic to be different, but I see no good reason for deliberately favoring magic over non-magic in raw power. If you do, you destroy the basic assumption of D&D that every party member is a valuable part of a team of heroes. And i consider that assumption to be far more critical to the basic idea of D&D than an imbalance born from accident and bad game design.
Your point of view seems to assume that a straight-up balance of power is essential to D&D, which I disagree with. D&D is a game only in the sense that some kids running around a playground pointing at each other shouting "pew pew" is a game. D&D is not competitive. No one wins or loses. D&D is not chess, warhammer, or even Baldur's Gate. It's *possible* that you could have a party made up of high-level adventurers and a low-level commoner and have a great time.
The rules are here to describe situations and help us to decide outcomes that are not immediately obvious or which we would like to leave to chance rather than decide ourselves. Balance is a secondary goal, albeit a meaningful one.
This idea of magical temptation has no precedent in D&D. Also, mages in D&D never become too powerful for their own good. They become too powerful for the campaign's good, or even for the game's good. Basically, it is everyone other than the magic-user who suffers in an imbalanced game.
Really, no precedent? As for that second assertion, it's a valid opinion but it certainly doesn't apply to everyone or even a majority.
D&D doesn't posit a capstone spell called "Wish". That's just one spell, no more essential to the game than a Hideous Laughter spell. If anything, it is one of the most poorly designed spells in the game, that pretty much causes a total breakdown of the spell system.
Wait, so D&D would be D&D without Wish? Not all spells are created equal. Wish is certainly more iconic than Hideous Laughter and is pretty fundamental to the classic D&D conception of magic.
I'm not sure if Fighters need to stop being Fighters at high levels. They just need to be allowed to genuinely become high-level Fighters.
Hey I'm all on board with that, as long as it doesn't mean that they get abilities arbitrarily limited to use per unit time or be able to do supernatural things. I'm not here to defend the 20 level progression of the 3.X fighter. There definitely need to be some changes there.
There are plenty of great examples of high-level warrior-type characters in the bast breadth of legend, fantasy literature, and pop culture. They range from low-level figures like Lancelot and Conan to warriors of
real strength like the Monkey King of Chinese lore (immortality through brute force, gotta like it). People need to simply open up their realm of inspiration and stop forcing Fighters to act like low-level mooks when they are level 17. Moving mountains are rivers is not "non-fighter", it is pretty much the definition of high-level Fighters of myth and fiction.
I think what you're talking about is within the realm of divine or epic rules, not the fighter class (though I certainly agree that these things are within the scope of what the rules should cover).
***
The bottom line is that the poll and many of the responses seem to assume that the classic D&D model (spellcasters attaining greater power over time than others) is "unbalanced" and the 4e model (everyone gaining similar powers with flavor difference and some substantive differences based on power source) is "balanced". 3.X and its predecessors and successors are balanced enough. They're balanced in the sense that all character types can be played and enjoyed, but not in the sense that every character can take on his equal-level counterpart in a g;adiator battle. Which is why the poll overstates the case. As many respondents have said, you can have balance without making everyone the same.