1. The generous death save rules and ability to re-enter combat. Often referred to as the "whac-a-mole" problem, there isn't a strong need for full healing in combat. In prior editions, if you were downed, you were usually down for the count; you needed healing long before you hit zero hit points. That's no longer a major strategic concern in 5e.
I feel that this is one of the big issues. getting back up from 0 is so easy it is counter-production to be full healing in combat... right up until that isn't the case.
Disintegrates, Specters, being surrounded by multiple monsters (either auto-attacker or intelligent ones who are aware you have popup healing and thus will finish off people while they are down). There are plenty of relatively situational reasons why you don't want to hover near 0 hp, and a system which treats that as the appropriate tactic can create a real fail-state of those situations.
This highlights a general trend I have noticed in TTRPGs well above just in the case of combat-healing: -- even though it is very hard to say no to power-boosts, every time you add X to both sides of the scale, it increases the tendency for the fail state to be catastrophic failure (instead of a character screwing up, it is a character dying; in stead of a character dying, it is a TPK, etc.).
In other versions of the game (for example, BX, where 0hp=dead), when your front line level 8 fighter gets reduced to 7 hp, they drop back into the second or third ranks and switch to spear or bow, and let the cleric, hirelings, henchmen, and monsters you've bribed to be on your side man the front lines (or the party ran, or whatnot). It isn't exactly heroic, but the game is balanced around the premise. As such, you had a strategy that worked in most cases (until a dragon came by and breathed on the first, second, and third ranks, that is). With 5e there is a lot more instances where the primary strategy around which the game (or at least in-combat healing) is balanced fails. At least unless you have a Life Cleric, Paladin, Glamour Bard or other character who can do a significant amount of burst healing or pull-you-out-of-the-fight effects.
All of this means that in prior editions, in-combat healing was often more required. Whereas in-combat healing, especially beyond the levels already in the base rules, will tend to make the game more crazy, because every single character already has a reservoir of an additional 2x hit points, per day, that they can heal without any extraneous help. Which is a bit different than when you were playing older editions.
I just want to point out that this is a mixed argument. In-combat healing isn't more or less required because of the additional 2x HP that is only accessible out of combat, or at least there needs to be additional points made to square that circle.
How would in-combat healing make the game (with 2-3x hp) more crazy? Through what mechanisms?
Monsters are giant bags of hit points. The party will wear down the hit points and triumph. They will take damage. After the combat, the party will heal up. Disturbing that balance by providing too much in-combat healing (and there already is A LOT OF OPTIONS FOR THAT!) begins to unbalance the encounters. As it is, most healing in combat requires choices in the action economy- between doing more damage to the giant bags of hit points, or doing less healing to a party member.
Again, I think there is an argument missing in the middle here. How will providing 'too much' (which perhaps we can swap out with 'additional' when appropriate) begin to unbalance the encounters? By what mechanism would making ICH be roughly on par with acting to end the encounter sooner unbalance things*?
*it certainly would draw combat out, which I think is a predominant motivation for the current arrangement, although the designers will never tell
