Idea of high number of low cr monsters being viable threats for high level PC's was good one. At least from the world building and in setting consistency point. It makes imaginary world feel more real and less video game like.
Problem is, for that to work, it starts to trickle into tactical combat (positioning, using covers and distance, high ground, multiple angles of attack, ambushes with overwhelming numbers etc) and edges on the dreaded "mass combat". It's also almost non existent in published adventures, at least from what i seen going trough them.
I would rather say: it is a good-SOUNDING idea. Like prestige classes, and skill points, and a host of other ideas, it sounds wonderful. It sounds like it gives you everything you could want and nothing you don't.
And then the practicalities of actual game design hit, as you say. The idea falls apart as soon as it makes contact with anything like real, experienced gameplay.
In other words, it's one of the few
actual "white room" things. An idea that sounds absolutely wonderful in theory, and which people will die on a hill for...even though
in practice it runs into constant issues and causes the game to become something other than what the designers wanted it to be.
Minions was one of the things i really liked in 4e. They had good idea and i still use it in 5e. Also, it makes sense that Ogre starts as Boss and ends up as minion. Monster power is same, but when measured against party power which grows over time, that Ogre becomes less challenging. For lv 1 party, Ogre is scary. For lv 11, fighter can probably solo it with some effort. For level 21, it's one hit kill. Minons are there to soak up hits and give main boss time to do his cool stuff. Action economy.
Certainly. Because minion (and solo, and various other things) recognize a fundamental truth:
The game is an abstraction, and that abstraction is not
and cannot be a 100% perfectly naturalistic mapping of absolutely consistent things,
while also delivering the experience that D&D claims to provide and which, in general, players are actually looking for.
Naturalistic design is wonderful when you can include it. You always should try, if you can. But the simple fact is, the threat that a particular enemy poses to the party
really does change over time. Something that really was a horrendously powerful foe, something you would run terrified from at level 1, becomes an absolute cakewalk...and a single numerical system, bound by so many other requirements (it must have low numbers, it must be very fast, it must not bog down on details, it must be easy to teach, it must, it must, it must!),
cannot 100% perfectly accurately depict that while still delivering, as the books say, "heroic" adventure.
Minions recognize that the actual threat posed by an enemy is, to a certain extent, relative to how strong the PCs themselves are. That particularly strong PCs actually do perceive a given threat
differently once they have grown too far past it. That all you need is a single strong hit, which--because hitting and damage are highly abstracted in D&D, of all stripes--is best represented by an abstraction of occasionally needing to make a few more hits, or an abstraction of an unavoidable minimum amount of damage (e.g. "a hail of arrows" meaning you're still struck by
something, or "a fiery inferno across the floor" meaning if you're in that space, you're taking damage, it just might be lesser or greater). That actually captures what it would FEEL like to fight an enemy you are now so far beyond, they're cannon fodder to you.
The whole point of game design is to capture some kind of
feeling, some kind of
experience, not to make a picture-perfect world simulator that is accurate down to centimeters and copper pieces. It's why economics has
always been a weak point for D&D, and results in trivially-generated nonsense if the players do more than casually glance at it. It's why to-hit and AC are a
terribad representation of the way actual melee combat works, why HP are so grossly abstracted that they completely flub almost every part of injury and healing (healing should be very slow, but total removed-from-fight injuries should be
quite difficult to inflict, keeping someone from developing circulatory shock
which you can in fact do with exhortations and shouting! is much more important than any faffing about with "curing" wounds which
isn't even a thing you do to wounds!, etc.)
That terribad representation of the actual process of causing, receiving, and treating wounds is, however, quite effective at inculcating an experience of
heroic combat--hence why it's used in D&D.