When people say that fighters can't do anything out of combat, or that a caster will always do better than a non caster, it relies on the assumption that the caster will always have the right spell ready, all the time. No one actually has to come out and specifically say that; it's inferred by the argument that is being made because otherwise there is no way that argument could work.
I think the root of the problem is that fighter's cant do anything other PCs couldn't already do.
A war cleric with 14 Strength and guidance is just as capable at overcoming Strength related tasks as a 20 Strength fighter without using a single spell slot.
A valor bard with expertise in Strength is better than the fighter at such tasks without relying on his spells..
A moon druid can turn into forms that are far more capable of overcoming physical challenges (stronger, faster, flight, etc) without relying on spells.
Even the wizard has access to rituals like find familiar (can fly, help, etc), unseen servant, tensers disk, and other spells that are more valuable to exploration without requiring a spell slot to be spent.
Warlocks have invocations like at-will jump or levitate.
And so on. Most classes have a variety of ways to contribute to exploration without using their spells that rival the basic skill checks the fighter has.
Because of bounded accuracy, even a 14 Strength PC isn't significantly behind the fighter in terms of capability in the field of STR based exploration.
On the other hand, the spellcaster might also have access to some low level spells that completely bypass an athletics check with 100% chance of success.
So what does the fighter contribute that any other 14 Strength PC can't contribute nearly equally well. Never mind the fact that the other PC probably also possesses exploration related abilities that are far more potent and useful than the fighter.
Rogues don't generally have expertise in Athletics, though, since Strength is a dump stat for them. In the off chance that you did have such a character, then you might get past the obstacle without difficulty, but the bad guy can't account for such ridiculously unlikely events.
And I have no idea how the NPC is supposed to create a lasting anti-magic field, but it shows up enough in published modules for it to be one of the go-to solutions for these types of situations. Or if not permanent, then make a contingent Dispel on anyone in that area. Or animate zombie birds to break your concentration. However you do it, though, it has to be cheaper than building an entire gate out of adamantium.
Obviously it's absurd to say that the fighter can't do anything at all out of combat. For instance, he might guard a doorway while the thief works on a chest, or hold a torch so the wizard can decipher some runes on the wall. Similarly, the idea that a caster will always do better than the fighter is a degree of absolute prognostication that can't ever be true. Afterall, a wizard might lose his spellbook or his voice or something, and do far worse than any fighter until he gets it back. Or a caster might be killed by kobold sling-bullets at 1st level and never raised, thus radically under-performing relative to the fighter at higher level.When people say that fighters can't do anything out of combat, or that a caster will always do better than a non caster, it relies on the assumption that the caster will always have the right spell ready, all the time. No one actually has to come out and specifically say that; it's inferred by the argument that is being made because otherwise there is no way that argument could work.
There is a base-line of things that just anyone can do in 5e, and bounded accuracy makes that baseline non-trivial. Making an ability check or an attack roll, for instance, are things literally anyone can do. You don't need even one level in a class or any proficiencies to make an ability check. Even if you're a 1/8th CR Kobold making a STR check, you'll hit a DC of 15 once in a while.I think the root of the problem is that fighter's cant do anything other PCs couldn't already do.
Why wouldn't a rogue take expertise in Athletics? Makes Climb, Jump, and Swim easier. Stealth, Perception, Athletics, and Investigation/Acrobatics as the fourth is what I usually take.
So we've gone from they don't have anything to do in non-combat situations... to they don't have fightery things to do in non-combat... to they don't have unique things (well except for Action Surge, Remarkable Athlete, Second Wind and Indomitable) to do in non-combat situations... and this is all ignoring the fact that they are top tier combatants as well... what was the issue again??
Rogues don't usually have high Strength, and they would get more mileage out of taking Expertise in skills that are keyed off their high stats - usually Dex, Int, Wis/Cha in that order. After all, the Fighter (or Barbarian) can probably handle any Athletics check that might come up.Why wouldn't a rogue take expertise in Athletics? Makes Climb, Jump, and Swim easier. Stealth, Perception, Athletics, and Investigation/Acrobatics as the fourth is what I usually take.
Even the abilities that are supposed to help outside of combat really don't do all that much. For example, remarkable athlete is a minor bonus to a few untrained skills and a slightly increased jump distance; hardly remarkable.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.