I find this a rather false assumption.
5e healing is a lot less "liquid" than any previous edition other than 4th. Editions 1 through 3.5 relied on healing through magic from specific sources, whereas 5e has a significant amount of healing on a per-character basis from HD. Compare to the editions where rest healing was a few points per day, where healing resources were actually pooled together.
Furthermore, in 5e compared to 4th ed (and 3rd ed in some cases), defensive-capable classes are much less "sticky". This leads to damage being spread around the party a lot more, rather than the Defender-type classes being able to focus much of the incoming damage upon themselves. In those editions, temp HP given to those classes would almost definitely be used, and thus count as "party temp HP", whereas in 5e there is no guarantee that the character you had just given temp HP to would be the one who would be taking damage. The Moon Druid's temp HP for example can't be considered the party's temp HP because the Mood Druid can't guarantee that she is absorbing damage that would otherwise be applied to the Sorceror for example.
The only scenarios where your base assumptions are valid, are those where each character in the entire party is taking constant low-level damage for long enough to seriously deplete all of them during a single encounter. This is a rather rare occurrence: it is more likely that someone will go down due to a bad position or bad luck/crits while some of the rest of the group are relatively healthy.
In short, 5e is probably the edition where those assumptions are the least valid.
All fair points. When I mentioned liquidity I was thinking of not just how easily healing resources can be converted into HP but the overall "accounting liquidity" of how that squares with incoming damage, or basically how easy it to manage party health and keep everybody alive. This is very easy in 5e. What really makes it easy are the rules for death and dying: high threshold for instakill, low likelihood of dying from death saves and heal-from-zero, which perversely incentivizes waiting for someone to drop before healing them. There's almost always a chance to save a downed character, often without them missing a single turn as long as the party has healing slots left. This is why characters almost never die outside of a TPK, and the party dies when they can't pop up KOed characters with spells (or forcefeed them healing potions) anymore. If the rules were death at negative Con+level, or simply dead-at-zero, then it would be much more difficult manage health across the party and characters would have to rely on their own defensive capacity without relying on a pop-up heal. Which brings us to the point of the thread: I'm concerned about character imbalances resulting from modifying the death and dying rules, because classes seem to have very different defensive capacities, even comparing classes of the same traditional role, like Barbarian vs Fighter and Druid vs Cleric.
Killing PC is not the goal of DnD. And even then they come back with an new pc.
DM should focus on mission failure/success.
I agree that problems would result if the DM made it their goal to kill PCs. That's why I want the system to do it instead.
Can you create a good challenge with a complex scenario and the sporadic, somewhat arbitrary use of simulationist game mechanics? Yes, but that's like saying you don't need a washing machine to clean your clothes.
As I said earlier I associate this kind of prep-heavy light gamism with Call of Cthulhu. It's nice to use a simple, "invisible" system for that style of play, rather than a 300 pound washing machine like D&D. You can focus on the atmosphere of the game and worry less about plot-busting abilities like Speak with Dead and Zone of Truth, and gamist constructs like XP and levels. I wonder how Curse of Strahd would run with Cthulhu Dark Ages.
My opinion does not matter in your games. The only opinions that matter are those of you and your players.
However, you will always have an undue influence in the outcome of a game when you are the DM. You make the rules, stack the deck, deal the cards and tell the players when they can play their cards. An interesting question: If this is a concern, why is it a concern? What does the answer to that question say about your group?
I don't think a DM must have an undue influence in the outcome of a game. I think many of the tools and techniques that help to minimize conflicts of interest in DM prep and refereeing (to maintain system "hygiene" as Eero Tuovinen would say) have fallen out of the game through the years, some of which are back in 5e but unfortunately presented more as oddities than serious tools, e.g. random encounters.
As for the scenario I described assuming success: To an extent, yes, and to an extent, no. We started with the premise that the foes in the adventure were not deadly threats to the PCs. Unless the Dice Gods are cranky or the PCs do something cringe-worthy, they'll survive. However, what happens if the NPC friend dies? If they have to kill townsfolk to get to the vampire? If the We're rat gets away the first time the PCs approach? This goes back to the concept of what is failure.
Well, that sounds more functional but less exciting. The fact that it's not obvious what will happen lessens the impact of the failure. With character death and start over at level 1, the impact is very clear.