Chaosmancer
Legend
There is a reason to go back to the 4e style of using hit dice to heal in order to provide better healing. In 4e, a healing surge healed you for 25% of your maximum hit points.
You could only use this once per encounter with Second Wind, or outside of battle, after the 5 minute short rest. Most spells that allowed you to heal gave you your healing surge value plus a bonus of some kind.
This allowed for a Healing Word to provide a significant chunk of hp, but it also limited how much healing one could receive over the course of a day. The usual argument about healing spells comes down to this-
*The ultimate limit to how much players can accomplish in a game day is their hit point totals. Every spell slot or ability that provides healing is in addition to your use of Hit Dice.
When you take these things in aggregate, only a tough encounter can really drain the resources of non-spellcasters in any real way. In fact, some non-spellcasters, like the Fighter, even have a resource to heal on their own, that is recoverable.
So consider a 4th level Fighter with a 16 Constitution. Let's say they have 36 hit points. To actually stop them from engaging in fights, you need to get them down to 50-75% of their total hit points. By themselves, with no one else, you need to consider 4d10+8 healing from Hit Dice, and their Second Wind, which is another 1d10+2 that can refresh after a short rest.
Then you have to take into account whatever healing they could get from their Cleric/Bard/Druid/Paladin/Ranger in the party, not to mention cheap potions of healing (in a game that doesn't give you much to spend money on) and the Healer Feat.
If you're the kind of DM who uses an attrition model for adventure design, even if you limit resting using grittier rules, or ban the purchase of healing potions, this is already a high bar to achieve in order to feel like you're actually draining resources from the party.
If healing spells get better without changing anything else, that bar might become stratospheric. And this is assuming you actually can fit in the fabled 6ish encounters per game day.
Anything that the players can do to rest more often also has to be addressed, since that gives them more ready access to resources.
I can't stand this model personally, I want healing magic to feel worthwhile again. But at the same time, I'm not going to force extra battles just to pad out my adventure, and I also like big setpiece battles that are tougher than normal (and thus, might require better combat healing).
But as long as this is the way 5e is built, we're going to get pushback even if we mathematically prove that combat healing is terrible, because there are DM's who are looking at daily resources players have, and already feel it's too much.
I'm sure if Divine Spark leaves the playtest intact, that's going to be one more issue for them, especially since they also seem to want players to regain all Hit Dice at the end of a long rest, instead of half...
There is a problem in your model though. And it is also a problem I addressed with a completely different rule. You assume that to stop the fighter you need to drain all of their Hit Dice. This is only true if A) You want to stop the fighter from adventuring and B) They have time to short rest during the encounters you are planning.
I have often seen players panicking because they have reached 50% of their hp, with full HD left. I once had a game where, as a Life Cleric, I had to insist that the party only take a short rest instead of long rest, in the middle of dungeon, because we didn't need to recover all of our resources. It isn't a matter of the actual danger, but the perception of danger for many players. And if they drop to 10% of their hp, while still having all their HD, they aren't thinking "this was a boring adventure, I still had plenty in the tank", they are thinking "Dang, that was close, one more hit and I would have dropped". Because it doesn't matter what the HD are, it matters how many hp you have until you drop and if you need to seek a place to hole up and heal.
I'm still somewhat baffled by this idea that every fight and every adventure should have the party on the edge of their seat, in fear of whether or not they can survive, no matter how prepared they are for the adventure. If someone brought a healer, then I'm fine with the party being less worried about dying than if they didn't. That seems like the entire job of the healer. But right now, most parties just rely on short rests and potions and only use the healer to pop them up. The healer isn't changing gameplay by being included. In fact, many times the healer doesn't bother to use their resources on healing, because it isn't worth it.
Now, I will admit, I have houserule that helps with attrition. It has rarely come up, just do to not having long-term games recently, but it solves the problem from a different angle.
When you Long Rest, you don't regain hit points for free. You spend Hit Dice as normal. And, like the original rules, you only regain half your HD on a long rest (before or after you heal). This means that my attrition (when I can utilize it) isn't one day of fighting. It is multiple days of fighting. Sure, you get all your spells back, but you don't get items and you slowly drain of Hit dice.
I'm tired and maybe not phrasing things the best, but this is where I'm at. I don't want potions and magic items to be the main healing. I don'teven particularly want Out of Combat healing to be the only viable option. I want in-combat healing to matter. Because that is when players are most engaged with their hit points, and where the healer WANTS to be the most effective. At preventing that drop to 0. Not at making the short rest more effective.