An alternative to gritty realism?

[Amechra]

We prefer a living world, so it really varies greatly how difficult the fights are. The players are definitely kept on their toes though, and regularly get into fights where 2-3 out of 5 are unconscious and the rest barely finish the fight. I'd say on average it's hard difficulty, so I guess it's maybe less that the martials "need" the hit points as much as they just "want" them as much as the casters want their slots back.

One possibility could be to send the party to somewhere a bit more dangerous, or let them get the wrong kind of people angry with them. The first full 5e campaign I was in was set in a city - we spent the majority of our time fighting the gangs/cultists who were preparing for a full-on elemental invasion of the city, who definitely held a grudge. Something like that could justify having fights more often.

Basically all playing casters now. 2 out of 4 of the old crew have tried martials and in no uncertain terms made it clear that they very much disliked it and went back to casters. One member of our group is new and appears to want to take a "martial classes exclusively" approach to the game, and I don't think she realizes the problem simply because she's new.

Ah, I've heard that song before. As someone that avoids casters in 5e¹, I can definitely sympathize.

¹ I'm not a fan of how casters are thematically vague in D&D. My preference is closer to Warhammer Fantasy's Winds of Magic/Priestly Magic, where your Wizard has some basic generalist magic and a small list of more powerful spells that you can learn. If you're a Bright Wizard (for example), your more powerful spells are going to all be variations on "set things on fire" or "inflame passions". If you're a Shallyan Cleric, you can cast healing spells and fire anti-disease-demon lasers (and you're the party's favorite person).

[opaopajr]

Encounters are meaningful learning situations, which *can* mean Combat.

Encounters tax resources: time, space, allies, gear, class features, etc.

You need WAAAAY more Encounters.

(And remember: not all encounters are worth XP. Routine or trivial issues may cost resources, but still learn you nothing new. Hence being selective about encounters while trying to escape life's busy-work encounters. Life's Maintenance is taxing, so TAX them!)

But you do not need more Combat Encounters. Add a lot of Explore and Social Encounters. Add them in different locations and different frequencies. This taxes the party's Space AND Time, leading to splitting up or extra resource expenditures.

You are not overwhelming them with a breathing world. You are handing them a manageable world. No believable world is manageable by a mere squad. Drown them in potential, lost opportunities, and chances to earn meaning. Tax ALL their game pillars -- ALL OF THEM!