My thinking is they expect too little from DMs. They constrain the number of foes because they think it makes games unplayable, while they ignore the slog of BagOHitPoint monsters. And when you add lair/legendary actions, its like adding more NPCs as its more DM actions, except you usually can't eliminate them on their own.
5e is kind of boringly consistent. 1e was wildly inconsistent. We need enough variation that its "spicy" but not so much that TPKs happen frequently
Thinking back to 1e, players hit harder. D8+4 Went a lot farther before hit point inflation. IIRC, 1E dragons inflicted (their current hitpoints) as their breath weapon. A 100hp dragon was a terror. Sure, each lightning bolt/fireball took off a big chunk of their life, but you had to live to land it and kill it before it breathed a second time. But when you did hit it, it got progressively weaker, which doesn't happen in 5e.
So can keep Legendary Resistances but IMO, replacing 1 Legendary/Lair Action with 2 mooks is about right (think half the CR of the boss). It starts off as more actions, but they should be cleared out before the GMs gets brain-strain.
Yeah, but it probably isn't moving away from the attrition design, either.
I agree, but this changes it from "one immutable foe you attrit" to "attrition of numbers, causing a change in the tactical landscape."
Mooks will often be in other rooms, sleeping, etc, so they (or the boss) may be 1-3 rounds later to the fight, which adds some more flavor. Yes, fights would take longer but it would not be a slog. And it gives bosses and/or mooks opportunities to escape. Recurring villains are fun.
And then plan for, and foreshadow, "mostly dead" PCs. I mean, 5e has introduced a number of "not entirely dead" recovery spells and mechanisms. Let "mostly dead" be something that happens every other boss fight.
I think hit point deflation combined with bringing back mooks/minions (lower level enemies that die relatively quickly but add tactical benefits and action economy changes) would make the encounters spicier but take off the slog because the action economy changes as mooks (and PCs) are dropped.
I'd also want to bring back PCs running away. It seems to be a lost art. That might me a bridge too far. My players know that sometimes, run away is the right course of action. Don't be a treant fighting a wood chipper with their bare...branches. Run away and come back with a hunk of iron you lob into the chipper.