Thank you Aribar. However, your case is one with a clear timeline.
The thing is any solution the DMG happens to decide on ought to work for very different scenarios as well. Such as
A. Wilderness hexcrawling. "Here is the valley of Doom. Go enter it and return when you are fully laden with loot!"
An example where the whole point is the lack of any story-driven time restrictions. This does not mean the game can't restrict your resources (such as hit points and spells).
B. Dungeon Bashing. "Clear out the Kobold Hive! Do it quickly, or all remaining kobolds will gather into one force - a fight you can't win!"
C. City Intrigue. "Your aim is to gather evidence against Countess Bathory. Doing so will entail going to many banquets and blackmailing many prominent citizens!"
D. Desert Trek. "Crossing the Mirage Desert will take months!"
All these adventures have very differing needs as to pacing and resource management.
But they all should be runnable using the same ruleset of D&D. Which, as we agree, runs optimally on 5-8 encounters per long rest.
And in one and the same campaign. No switching out the DMG in between sessions!
I'm arguing the obvious solution is for the DMG to simply state how the D&D engine runs best - what the designer expectations are.
Then:
D. Assuming there will "only" be 5-8 monsters to encounter in the whole desert, state that no long rests will be possible in the desert at all. There are two oasis, and each will afford benefits equivalent to a short rest.
C. In this case, the rhythm of social and combat encounters can be impossible to predict. Perhaps your employer Mr Darcy hands you three special cards allowing you complete safety at a safe house. Meaning there will be no more than three long rests or your mission will be exposed/a failure.
B. This is what the game handles per default. You would think. Me, I would say it needs 5 minute short rests and 1 hour long rests, but still, the option is in the DMG. Point is, this is the only scenario for which the DMG is sufficient!
A. I would say that unless the Valley is so full of monsters you can hardly move between random encounters, you will have to abandon the resource management minigame and make every battle individually challenging. OR, you can boldly say that for this scenario you simply need 5+ encounters or you simply won't get any benefits from taking a long rest. An abstraction, sure, but one that allows the adventure to work as a GAME as well as a story.
Obviously, many gamers are happy to ignore the expectation of 5-8 encounters. And that's fine. But also beside the point - my argument is that D&D isn't helping me uphold the 5-8 encounter balance that means I can have "easy" encounters that aren't made completely trivial and meaningless (from a challenge perspective - the story angle is something else).