3. So, the solution? Make the DMG direct DMs to lowball encounter difficulty. Take your XP budgets per encounter, and cut them by 40%-50%. Then tell them to run double the number of encounters. Now the PCs are more likely to have opportunities to short rest before they feel they must long rest.
The trouble is, 6-8 encounters is boring to play, takes more time at the table, and takes more DM time because you just have to create a ton of encounters. It's just inconvenient for the adventuring day to have so many, which I think is clear because here we are 8-9 years after the game first released and people are still complaining about it. 3-4 always felt more natural at the table. You also run into another problem: Boss fights and traveling days where you often only ever want one encounter in a day, and how do those work with short rest classes? Worse, the PCs might still choose to long rest before short rest classes get their full benefits.
While not necessarily disputing anything you wrote, I would say that dungeons play a very significant role in the design, even though they seem to be given short shrift in these discussions. 3-4 encounters per day sounds good if you're talking about a plot-driven adventure, or a city-based adventure. It would be far too low for a dungeon-based adventure.
I'll also note that people act like 6-8 combat encounters is this crazy, impossible number. Yet Lost Mine of Phandelver, probably the most well-received and universally praised adventure put out by WotC for 5e, is very much along those lines. You have:
Day 1: Goblin ambush and Cragmaw Hideout (6-7 combat encounters)
Day 2: Redbrand Hideout (7 combat encounters)
Day 3: Ruins of Thundertree (9 combat encounters, not counting the dragon!)
Day 4: Cragmaw Castle (9-10 combat encounters)
Day 5: Wave Echo Cave (10 combat encounters)
Certainly, not of these encounters will happen, nor will all encounters that do happen necessarily lead to combat. But certainly this is proof of design? It may not be what everyone wants, it may not even be the playstyle in fashion 8 years into 5e's life, but I don't think it can reasonably said that WotC's math was unrealistic, or even boring and out-of-fashion given LMoP's reception.
You also run into something in 5e that I don't really recall happening much before: arguments about resting. Half of the part will want to short rest to recover abilities, and the rest will see no benefit so they don't want to. It's a strange dichotomy that encourages player vs player conflict. That isn't a good outcome.
I'll certainly not gainsay others' experiences, but this has never been mine, among 3 or 4 different groups. The short rest is so short and unobtrusive, and yet beneficial to the entire party, that even if it wasn't handled as part of passage of time between set-piece encounters, I've never seen any arguments. Someone suggests a short rest, and everyone's like, "Sure. I wouldn't mind rolling some hit dice." My groups have all tended to just naturally fall into the pattern of 2-3 combat encounters, short rest, 2-3 combat encounters, short rest, 2-3 combat encounters, long rest. With variance depending on difficulty, of course.