Wow. Attribution.
That's not actually /solving/, and, really, 'manageable's what every classic edition (and even 3.5) did, already, anyway...
Not actually what LFQW refers too, though it's closely related, and, well, it's a good description of an important issue that comes up when running /any/ edition of D&D, so worth talking about (more on LFQW later).
Yes, no-casters have generally tended to grind out their contribution (little more than dishing out and soaking up damage, in the case of the classic 'simple fighter') in a consistent, round-by-round, entirely at-will manner. And the idea was that, while casters would completely upstage that any round they cast a spell, they wouldn't be casting spells all the time - because there were a /lot/ of considerations, limitations to casting in the olden days.
[sblock="it was hard out there, for a caster"]A caster had to memorize spells in advance, so he had to pick not only the spells he'd likely need, but how many times he'd need each of them. In practice, given imperfect information, that meant needing spells that you could have picked but didn't, and either casting a poor substitute or not casting at all in the round that came up; that meant not needing a spell you did memorize, at all, so ending the day with it uncast, essentially reducing your spells/day by 1; that meant hesitating to use a spell early even when that might end up having been the best chance to use it, even hesitating to cast a spell only to find the day's challenges over and, again, yourself functionally 'down' a spell/day. Casters also had to deal with difficulties casting spells in melee (or even at range or under even slightly challenging adventuring conditions, like a pitching ship's deck or swaying rope bridge or back of a moving mount) and could /lose/ a spell they attempted to cast as a result - again, down another spell/day.[/sblock]
Later editions progressively did away with basically all those restrictions. Casting in 5e is carefree, wasted slots are rarity, all your slots are likely used as efficiently as possible, limited only by your tactical acumen and DM's fiendishness.
What's more, the baseline that casters operated at has edged up, too. A wizard throwing darts in 1e, in 3e, was shooting a light crossbow, and, in 5e, throws attack cantrips that scale with level.
It's no surprise that the prescribed day length to balance the Champion trudging along, grinding out the damage round after round, punctuated maybe every-other encounter by an action surge, with the Wizard, casting his best spell for the situation with every slot, and tossing one of several attack cantrips in between, required 5e prescribing a 6-8 encounter, 2-3 short rest 'day.'
Thing is, that's not a solution, it's an issue, in itself, as it's dictating how the DM run his game and what decisions the players have on the table. Because, if the players /do/ decide to long rest more often or short rest less often, the values plugged into that formula change, and the putative 'balance' vanishes. It's not like 5e doesn't give players tools to carry through with such a decision, either.
So, saying it's 'fixed' or 'manageable' is pushing it. It was manageable the same way it was in the TSR era. Heck, a draconian* enough DM could even tax 3.x casters so heavily with time pressures, misleading information, and brutal enemy tactics as to drag them down to the Tier 5 classes' baseline and impose heavy-handed balance, even on that 'peak magic' version of D&D.
So, yeah, time pressure and encounters/day and restrictions on magic have always attempted to impose balance on innately unbalanced classes in D&D.
LFQW, though, actually refers to the progression of classes as they level. A classic beatstick fighter increments his chance to hit every level. He might get a better magic weapon now and then. He eventually gets more than one attack per round. His DPR steadily increases. That's the LF. The caster, OTOH, starts out with 1 spell of 1st level cast at 1st level ability and as he levels gets more spell, higher level spells, and all those spells scale, so his power balloons at an accelerating place. That's the QW (Q for quadratic, and it might be better to say geometric or hyperbolic or something else, depending on how big a math geek you are and how precise you need your metaphors to be). You might think that a 5e caster's spells scaling some things, like damage, with slot instead of character level addresses that, and it does, a bit - but, then, so did 3.5 scaling save DCs & capping damage with slot rather than caster level, and that didn't help much, either. (Bottom line, though, low level spells stay useful in both eds: 3.5 for spells without saves like utility spells, 5e for spells /with/ saves. The coefficients are slightly different, but it's still metaphorically "quadratic.")
* cf: Heart of Nightfang Spire.
