And if the DM tries to scale up threats to cope with the lack of attrition, he might find that the party are kinda glass-cannon-ish, especially if a few rolls go against the party.
I'd like to gently use your comment as a point about why the 6-8 is so often ignored. None of this is meant as disrespect or aimed particularly at you.
Increasing the deadliness of an encounter while reducing the number of encounters may balance HP attrition, but not resource attrition. That 6-8 is a quite good balance point between the long-rest recovery model characters and the at-will characters in tiers 2 & 3 of play. (Tier 1 the long-rest have a lot less and rely on cantrips, which are balanced as weaker but not greatly so against the single attacks the martial have at that tier, so it's not as important then.)
Easy example isn't a caster, it's a barbarian. A barbarian with 3 rages who knows the DM normally throws 6-8 encounters a day at them will not rage in every one. They will have encounters - half or more - without rage. When the DM throws 2-3 encounters regularly, the barbarian can count on their rage in every one.
Spells are pretty easily shown. If a level 3 buff lasts all combat, how many slots does it consume when there are 3 combats in a day? When there 8? How many extra slots for other spells does that leave?
Plus encounters are often made deadlier by either more foes or by more powerful foes.
Both of those options favor casters. More foes means that area of effect spells are more often effective. Instead of a fireball catching 3 foes for 24d6, it catches 5 for 40d6. Wow. For the more powerful foes, you need to think abotu saves. A foe will only have 2-3 decent saves, between proficiencies and just great ability scores. That's means that reasonable spell selection has plenty of poor saves to pick from. You DC goes up with both your proficiency an dyour ability score, those bad saves do not - it is just as easy to make a powerful creature fail it's saves as a weak one. So it's just as east to affect them with a crowd control or debuff, but that slot takes out more powerful foes then in a regular encounter in a 6-8 encounter day.
To bring all of this home, you can only jack up an encounter so far before you risk really exceeding the HP attrition. But that level of encounter is not enough to provide the correct amount of resource attrition. So there's more resources per encounter. Making everything easier. Let me repeat that -
as long as you don't push past the point were encounters will definitely kill characters, jacking up the difficulty and reducing the number of encounters is actually making the day as a whole easier than with the recommended 6-8 encounters.
That's why I'm saying that 5e is only easy mode because the design balance point and the commonly played point (by me as well) don't match. If you play to the design balance point the game is not easy mode.
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