Fighter's problem is out of combat they rely on a baseline competency in the ill defined skill system, with a few extra ASIs for feats or whatever, assuming they are not using them on their combat ability, something actually notable rather than patching their rather glaring non-combat weaknesses.
Fundamentally a huge part of these issues is that so much of the game has traditionally been combat, spells, and universals (everyone can do X and is constrained by Y, etc.). Skill systems have been vestigial (or in the case of 3e-rigorous, integrated, and weak*), and many to most OOC tasks don't have specific resolution mechanics. Those that do tend to have serious chances of failure, serious consequences of even attempting, and a handy spell available which can solve the problem without a failure chance.
*with some notable exceptions
I also think wizards tend to look better on paper than in practice. Having to actually pick what spells you get on level up, and acquiring others in game rather than simply rattling off the perfect solution, and having to make correct decisions in play instead on having the ability to look up monster stats and think things through at your leisure all contribute to making wizard look better than it tends to be in an actual game. Mind you, I do think wizards tend to outpace fighters, but in practice the gap is usually not as big as theorycraft tends to show in my experience.
This too -- although I think this discussion is often really 'casters (and paladins) vs. the rest,' instead of really 'fighter vs wizard,' and that changes the calculus a bit. Honestly my recent clockwork sorcerer seems to be better at being prepared, as I always have 2 spells per level which devoted to transmutation or abjuration* spells. Thus I have knock and spiderclimb and dispel magic and so on when we run into the situation where they are needed.
*of which I'm limiting their free slots to anti-magic and similar, not shield and absorb elements, etc.
Why, though?
Why not just let the characters do - assuming some collective wisdom among them - what they would logically and reasonably do; that being to nova at the first major encounter, wipe it out, loot the fallen, and then GTFO for the day?
Not the person you're quoting, but for me the answer would be, "One should, but then one should have a system balanced around that assumption (or, if preferred, modify the system such that that isn't what they would logically do)." If you pack three candy/power/granola-bars per hike, because you assumed they would be all-day events, and then find a shortcut such that they only take two hours (whereupon you start a new hike with more restocked supplies), you can: adjust the number of snacks you take, hike the longer route regardless of obvious short cuts, or get fat. Players aren't 'trying to get away with something' or something, but they are getting something that was intended to be given to them based on an situationally invalid assumption, and thus one can enforce that assumption*, alter how much is given, or end up with whatever negative consequences not modifying either invokes (in this case inter-class imbalances and player-challenge imbalances).
*If not however Helldritch is doing it, by the 'resting means ending the play session' or 'the opponents/treasure will flee/regroup if you go out to rest' method with which you and I both started, I suspect.
Is there a difference, when players demonstrably fail to conform to the designer's intent? Seriously. If you get caught up on designing a game for a behavior pattern that simply doesn't occur often enough to be relevant to the typical group, it seems to me you have fallen into a trap of thinking people should behave in ways that they simply do not.
It isn't just "what the most vocals are asking for." They've conducted their tests and realized that the expected balance point and player/DM behavior, in general, does not match the actual player/DM behavior. It is, effectively, exactly the same error they committed back in 3e, just to a (much) less egregious degree. They designed a game expecting people would play it the way it was playtested, rather than designing the game based around how people actually choose to play.
If you do, in fact, get approximately 8 combat encounters a day, then even the Champion rises up to the point of being maybe-kinda-sorta on par. And if you do, in fact, get 2-3 short rests every day (leaning toward 3 but not always 3), then Warlocks can keep up with other spellcasters (e.g. at 5th level, fresh day + 3 short rests gives you 2*4 = 8 spells that are always upcast to 3rd if that makes any difference; whereas a Wizard has four 1st, three 2nd, and two 3rd, plus three restorable spell levels via Arcane Recovery, meaning the Wizard has more spells but the Warlock has, in theory, stronger spells.) The problem is, many groups simply don't do that.
From all data that has been available to me--and, based on Crawford's direct statements, this is at least loosely like what WotC's much more official data shows--most groups are closer to 3-5 encounters a day favoring the low end. Further, they get 1-2 short rests per day, again favoring the low end (to the point of sometimes getting no short rests at all before a long rest.) Hence why they're shifting things. Players simply don't play the game the way the designers expected them to, and as a result, things are skewed to a point that the designers consider it a problem. Using my above example, the 5th level Warlock would typically get only four spells a day, whereas the Wizard still has all 9+ (anywhere between 10 and 12 total), and having a great cantrip option just doesn't quite compare to being able to drop two and a half times as many potentially combat-ending bombshells.
Also? "I don't have problems" does not mean "nobody should have problems."
Not the person you are asking, but for me it is this: Yes, WotC screwed up on what kind of gameplay is most representative amongst the players they ended up having. We got it. Everyone gets it. There are people who have been dead since 1873 who somehow have heard about this issue. I used to have this great big speech about how playstyle distribution is bi-modal (1-4 encounter days and 10-20 encounter dungeons), and choosing the valley in between those two peaks was foolhardy, until I realized there was no one who didn't know that. Now what?
I mean, EzekialRaiden has it right -- if you don't get the expected amount of encounters, short rests, rounds, situations where a spell might reasonably be needed if playing a caster, and so on, the balance shifts towards those classes which are mostly launching platforms for X/Long Rest ordinances and away from classes with more short-rest abilities or who rely on at-will or always-on qualities. Great, now what?
One can do the Helldritch option and mold the play experience to conform to the designed-around playstyle (or use the printed-right-there-in-the-
DMG optional rest rules which accomplish much the same thing), do the Lanefan option of letting players decide how to proceed (and presumably not have a lot of people wanting to play Champion Fighters or Thief Rogues -- at least if balance/contribution is a concern), or... I guess I'm not even exactly sure what you would propose people do.