I still don't understand why this problem keeps coming up.
If whack-a-mole is the problem -- and, to be clear, I've only seen it a few times in 5e -- why do you allow in-combat healing at all?
Because some people want to play healers, and do so in combat.
Or because sometimes you roll bad, and don't want to be out of the rest of the fight.
Or because you want those tense story moments where your need to rush to a dying ally before they succumb to their wounds.
If it's just Healing Word, why aren't you banning or modifying that spell?
A modification such as by spending hit dice?
What hill are you dying on?
Doesn't matter what hill. We can just stand up again. Dying isn't very threatening.
If you think the game is broken, why are you working around the problem instead of fixing it directly?
Not sure why you don't see this as an attempt to fix it.
Spending hit dice could prevent this from being spammed, but still leave room for it to work on occasion.
What I really don't understand, though, is if you're the DM and this situation keeps coming up that often, why haven't you considered that your encounter balance is wrong?
No. Because the best way for
players to heal is to wait for someone to be unconscious.
Why are your encounters so consistently bottoming out PC hit points that this is a strategy your players employ so much that it's aggravating?
Because they don't waste resources healing before they drop to 0.
For instance. If your ally is at 10 hp, and will be hit for 20 damage, 1st level healing word for 7hp, or a 3rd level healing spell for 17hp. What do you do?
You wait till you drop, and then heal.
That really shouldn't be happening. And even if it does, why isn't "the cleric burns a spell slot and is limited to cantrips and weapon attacks" not attrition enough?
At very low levels, yes. Spending one of your 2 slots to save someone from death is a good deal.
But at mid levels, you can do it 10 times, and still cast your high level spells.
A level 1 healing word will negate a level 20 hit. You just need to drop to 0 first.
Why do the NPCs have to "play fair" but the rules should punish the situation?
Why not give it a try yourself? Have 2 NPC, each with 10 healing words, and a few other high damage creatures.
Don't think it would be as fun as you think.