I suspect the tag on this thread should be changed, as this isn't a 5E issue. It's one that D&D has struggled with for a long, long time
Right. Fundamentally from almost day one*, but certainly by the time supplement I came out and magic users got level 7-9 spells**. That said, when magic users were eternally very fragile (and once AD&D added spell fizzling on a hit), and fighters got armies and preferential magic item (which let them be at least partially thor-like demigods without it breaking anyone's immersion), it worked better -- although it still was predicated on different conceptions of balance compared to what we now use.
*IMO, oD&D missed the ball by wildly reducing the power of higher-level fighting men (Heroes and Superheroes) as compared to how they fared in Chainmail instead of taking 8x as many hits as a level one fighter, a superhero only dropped when taking 8x as many hits at once, and also doled out a similar amount of damage in attacks (against the primary opponents on the field).
**which mostly were for NPC opponents because 'PCs were never going to get that high' or something like that. That notion was quickly dispelled, but new balancing metrics weren't put in place once EGG et al. realized that playstyle wasn't going to happen.
That era of play is also where we did get some of the issues with how characters were conceived. Despite allusions to Conan, Fafhrd, or Cugel, the initial game was built around regular joe soldier types (not appreciably distinct from general hirelings) wandering mazes searching for treasure. And that's where a lot of people formed their preference for how fighters are supposed to play out in terms in no-innate-superhuman-ness (even though the magic users have changed significantly since then).
Lately I've become convinced that the root of the LFQW problem lies with the central mechanic of D&D levels: that a person can become infinitely more powerful just by practicing. This works just fine for casters, since the basic idea is someone who gains immense power through knowledge. A character knowing a thing implies a time when they didn't know it yet, so it's easy to picture a wizard at Level Elminster or whatever and work backwards to imagine his earlier, lower-level days.
Most other powerful fantasy figures don't get their power in the same way. Hercules didn't learn to be the son of Zeus; Nightcrawler didn't learn to be a mutant. Achilles gets dipped in the river Styx and Bruce Banner gets caught in a gamma bomb, but it happens all at once, not over the course of several years and a dozen levels. None of these characters really have a level one incarnation. A suitable martial archetype for dungeons and dragons doesn't just need to be Hercules. It needs to be a nobody who can pick up a sword and become Hercules one day just by working really hard, and that's not something you see in Western fiction very often.
I don't know if the magic origin is important or not (people keep mentioning that Hercules is a demigod, and I keep wondering why that matters so much). But yes, a character must be able to pick up a sword and eventually become a character of whatever superior ability one lands on for the upper limit of fighter epic-ness.
I get the feeling everyone got used to the 'linear warriors, quadratic wizards' trope and now when you deviate from it people get upset.
The issue is them insisting on others play the BMX Bandit so their Angel Summoner can feel special. It's crappy and selfish and I just don't have any patience for it anymore.
I think you underestimate the fans/D&D gamers. I don't know a single person who plays casters that think it is a good thing that this disparity exists or want to keep martials down 'so they can feel special' (honestly, the people most avid at keeping martials constrained seem to be people who want to play very mundane martials). Instead I think it is a fundamental issue that caster stock rose when their limitations* disappeared, martials did not get a commensurate boost at that time, and most things that have been put forth to boost martials** violate someone's sense of what a fighter is.
*and I do think it is a good thing that spells are no longer limited by being really inconvenient to use, although exactly then is when they should have reigned in the overall power of the spells.
** such as giving them magic.
Saitama doesn't need that wuxia crap. He just needs one serious punch.
Martial prowess isn't to be found in complex tricks, nor is the problem in the power level between casters and martial classes primarily a combat problem.
Certainly that's the part where (effectively) the martials can't even play while casters readily can. That's why I think the best/easiest/most fruitful place to address this issue is make non-spell solutions to problems other than 'there are enemies around who still have hp left.' It's very hard for a fighter or rogue to solve the same problems as
Fly (depending-- is 'gets a pegasus mount' too magical a class ability?), but most of the other utility spells it could be easier to bring the spell solution and the mundane solution closer in effectiveness and rigorous, reproducible resolution mechanics.
