Sorry, why new DM here. I'll assume it is new DM time because the DM did something inappropriate.
Is it inappropriate for a hostile horde of 1000 orcs to exist?
Is it inappropriate for a DM to say, when players decide to charge a horde of 1000 orcs, to say "you all die"?
Is it inappropriate for a scouting force of dozens of orcs, if not dealt with, to be followed by a horde of 1000+ orcs a week later? Or that PCs defeating the scouting force and reporting the invasion being a possible solution to the problem?
That is logistical play and again, you're going to get ground down no matter what. It's the same amount of 'consequence' as the trolley problem.
Do you mean "if the PCs face an infinite line of foes, each capable of harming the PCs, they will eventually run out of resources" as "you're going to get ground down no matter what"?
In a given fight, the degree to which you use or lose resources will vary depending on what the characters can do and what they do do. You can choose to use 3 fireballs to kill a dozen rats, and you'll win the fight -- but you'll use a lot more resources than if you just stomped on them, even accounting for the rats sometimes managing to hit with a bite.
Entire classes in D&D are built around that assumption -- that you have a pile of resources you have to allocate over multiple encounters, and pick a good time to use the big guns and when to not use the big guns. Every single daily spell slot spellcaster has used this pattern since the game wasn't even called D&D.
In any case, my point is to use actual D&D mechanics.
Instead of using the basic unit of a D&D adventure as an encounter, make the basic unit the adventuring day (or, as I call it, the chapter).
You build Chapters. Adventures consist of Chapters. Chapters consist of Scenes. Scenes consist of Encounters. Encounters consist of Monsters.
(This is a simplification; all also contain terrain, reasons, motivations, etc.)
Chapters take a long rest to recover from. When designing a Chapter, the consequences of failing the chapter or ignoring it should be included. Why are the adventurers engaging the chapter? What happens if they don't? Why don't they take long rests in the middle of it?
Scenes takes a short rest to recover from. Similar questions apply, but here we have short rests instead of long delimiating it.
Sometimes you have a collection of Scene or Chapters with a timer, so no more than X short rests total, but let the PCs allocate the rests as they wish.
Encounters contain monsters. But you also have to think "why would the adventurers engage the monsters"? What happens if they don't engage the monsters? What happens if they start a fight, and they say "nevermind, I don't want to fight anymore"? Can they fight just one of the monsters in the encounter and ignore the rest - what makes the monsters "part of the same encounter"?
Sometimes you have one pool of monsters, and some possibility for PCs to split them up into different encounters or fight them all at once.
And on top of all of this, you can choose not to model the world in a PC-facing way and just have creatures doing stuff. But then you should expect the PCs to die a lot unless you are fudging it, because the kind of thing that PCs are doing in a realistic world results in character death. Sort of like how fighter pilots in an active war had average lifespans measured in hours.