This seems rather dismissive of the problem. Just because you have not encountered it does not mean it does not exists and suggesting that people who do are just old grognard yelling to kids to “get off their lawn” is unhelpful.
As mentioned above I happen to be an a position to directly compare the effect of this on the game on the same players and have noticed a definitive shift in the game because of this. You can also experience it by switching to somewhat harsher systems such as Savage worlds which has slower natural healing and low and behold you will see your players suddenly coming up with all sorts of creative way to avoid direct confrontation because of the harsher consequences, especially for smaller, less critical encounters.
I agree with you that with a structure of 6-8 encounters per day , the healing rules work pretty well, but that structure is difficult to always respect in practice, unless all you do is dungeons. Also it’s very difficult to make smaller, less dangerous encounters matter because all of the ressource attrition is mostly gone, hp, fighter maneuvers, even spells via arcane recovery etc being regained quite quickly by a short rest immediately after, which again encourages just dismissing with said encounter by a quick combat rather than engaging with the story, unless of course you make all of these incidental encounters deadly but this has its own problems ...
Funny, i thought the comment about how "lots of" was pretty dismissive in its own way.
Now my group is not yours and only 2 of my players have been with me since our first "dungeon" in 1980, but still i think i have a little bit of reference for observing the differences that systems have on players.
So first thing first, in *many cases* a system is chosen because of how well it reflects the "setting". usually a system with slow healing has that feature because it reflects a setting where that makes sense and usually that is also reflected in a lot of other things - often that includes fewer healing resources, fewer major boosts to defenses and the like - in other words a grittier and overall more dangerous system.
So, yes, when we have played more "gritty" scifi, low magic fantasy, cyberpunk (non-anime) spies and horror style games then in those games due to both the plethora of rules **AND** the setting expectations itself, the players chose different types of approaches, different preferences for "wade in" vs talk, etc etc etc. But the key thing is it wasn't "the slow natural healing" that was the culprit but the lack of lots of resources for healing, often damage related consequences etc.
Switch the same player to high end or golden age supers, high fantasy, high magic fantasy, anime-cyber-punk etc etc. and again different behaviors and such but not because of the "natural healing" but the totality os the system and setting and expected archetypes and tropes.
This is where i have had so much of an issue with the idea that just changing the natural healing rule creates some major change in behavior instead of just changing the optimization strategies. there is still easy ways to get to "quickly healed after a long rest."
Thats where i also take issue with whether or not this "adventure resource" thing is new somehow... its been around and a difference in style and play since the earliest days i was running games and seeing other groups play. To me we are not "lots" now but are where we always were "some" and it is dependent on style.
As for 6-8/d, absolutely - that is just aht standard model they used to balance rules around and is in fact not a panacea. What it does provide though is a "baseline" a Gm can use to evaluate how his game difference and where he might need to switch things up.
In a lot of different threads and discussions you have GMs who observe that it does not have to be 6-8 but can be 4-5 and still provide the same resource hit *if* they adjust upward the encounter difficulty. two difficult encounters then short rest then another pair slightly tougher with a short rest and then a bigger finale can work well as can many other structures with fewer encounters.
But the key is the GM has to present and structure the challenges in such a way as to through the story create the need to keep moving.
of course this should be mixed in with the opposite - cases where you do run into only one resource drain that is big...
one of my personal faves is a case where the first "leg" involves a BIG BAD (sometimes *the* Big Bad) which really drains the resources quite a bit and is often quite a surprise with added tension.) The the dead big Bad leaves a huge flurry of activity where if the PCs decide to take a lot of downtime they lose out on a lot of objectives and goals so their *hurry* may be to "rescue the captives before the others realize the jig is up" or "stop the lieutenants from getting away with the goods" and so on and so forth.
To me the part of the flaw in DND5e as far as this goes is not enough info about how to adjust and when to adjust away from their encounter path. Whether its how its worded or what or maybe a holdover from 4e there seems to be too much of a sense of "stick to the CR" even without noticing how far off standard you are.
It would have been great to have a few "benchmarks" provides possibly in the form of several "combats" set pieces or even just estimated damage levels.
That said, to me if you are aiming a game at a broad audience not just experienced GMs, having the system balanced around classes with abilities spread about thru "short rest" and "long rest" balances and expecting the new GMs to be able to manage that is not the smartest choice in my opinion. it would have been more likely better with either "long rest" or "short rest" as the mostly universal class ability time frame (perhaps with exceptions for each class at new tier abilities.)
But enough rambling about "shoulda coulda woulda if i were's"... I suspect that even in your experience with the same players, like i with mine, there were actually quite a few differences that added to the "option preferences" from one setting-system-expectations trio to another.
i also still believe the "problem" of "lots of players" expecting to enter each scenario fully healthy or some sort of OMG CRIPPLED backlash is more a matter of differences in expectations between the game the Gm is running and the game the players thought they were getting.
But to me it keeps coming back to getting the
RPG TRIO setting and the system and the expectations on the same page... as it always has.