This is certainly true in 3.5, but I don't think that it's an inevitable feature of the fantasy genre. Look at wuxia. Look at mythology. Look at what "peak human" means in the DC universe. I think that "strength via muscles" can do some pretty amazing things if the system allows for it.No, the problem is that the limit one can achieve with physical brute force from a human body is low, very, very, very low, so obviously someone pursuing strength via muscles is not going to get far.
Everyone likes to compare Gandalf to a low-level 3.5 wizard. That's fine; let's say that he's a Wizard 4. But let's further stipulate that Gimli is a Fighter 4. We know what a Wizard 10 looks like; what should a Fighter 10 look like? A Fighter 20?
Psyren speaks of a "plausibility gap" that should keep any mundane fighter from competing with a high-level spellcaster. But plausibility is subjective. I can imagine a setting like the Lord of the Rings where everyone is pretty low-powered. I can imagine a setting like the Wheel of Time where spellcasters have absolute supremacy. And I can imagine a setting where everyone gets nice things. I've heard it said that high-level 3.5 characters are practically superheroes; if so, then let them be superheroes. Let the fighter parry a stunning ray with his sword. Let the monk jump fifty feet straight up. Let the barbarian charge straight through solid fog or a stone wall without slowing down. Let the ranger spot enemies hiding a mile away. Let the rogue slip through a wall of force.
There are several different arguments happening in this thread. Should 3.5 classes be balanced? That's a matter of opinion. Are 3.5 classes balanced? Obviously not, but if someone still disagrees after fourteen years there's not much point to rehashing it. For my part, I want to play a high-powered system (or I'd play E6 or 5e), I want to play spellcasters and nonspellcasters in the same party, and I want class balance. If you want something else, then that's fine too.