Assuming soldiers are canon fodder for monsters is 1) treating them the same way shitty politicians do: as expendable resources and not people and 2) assumes a Witcher-like world where only the special few can defeat the monsters. That works in The Witcher. DnD is not The Witcher

Soldiers can use tactics. Soldiers can gain levels. If they couldn't, the Veteran wouldn't be a CR 3. The Champion is CR 9. The War Priest is CR 9. The Warlord is a CR 12.

Are they ALL retired adventurers?

So why wouldn't you send soldiers to clear out a dungeon?
* Adventurers are cheaper.
* Adventurers are foreign so seen as more expendable.
* You're at war and can't spare the units.
* Something else, be creative.

DnD can do a lot of things, but it can't do everything and it shouldn't have to. If you want to play Warhammer 40K, play that instead. If you want to play The Witcher, play it. If you want to play Nekomancy, please do. Modding any of those for 5e is an exercise in absurdity.

I haven't read this yet, but I'm digging the hell out of the concept. https://twitter.com/AjeyPandey/status/1297551139379052549

Oh, and b/c I suck at self-promotion, I mentioned Nekomancy upthread. If you haven't heard of it, it's a game I wrote where you use a tarot deck to cast spells, and also you're a cat. https://angrynerdgirl.itch.io/nekomancy 

So if you got through all of that and still want to explain the army to me, here's a man talking about this exact same thing. Maybe you'll listen to him instead. https://twitter.com/POCGamer/status/1297566433967104006?s=19

Since people are still explaining both DnD and the military to me, here's yet another military dude talking about the same thing I just said, but maybe it'll sound nicer when a man says it. https://twitter.com/Thrawn589/status/1297945951076782080