This topic has come up a lot and I don't have a new opinion to share, except this: The notion that 5e is so precisely balanced that it will "break" without 6-8 encounters per day -- regardless of factors such as campaign style, player skill, party composition and enemy capabilities relative to party strengths and weaknesses -- is pretty funny.
It wouldn't be fair to just run 6-8 encounter days and expect the game to balance perfectly, no. The DM still has to put effort into it, if that's what he wants. But, it's the theoretical point where those efforts will be at their least herculean, FWIW.
It's a guideline -- a rough guideline of dubious utility, in my opinion
Won't argue with that.
Yes, a reduced number of encounters increases the effectiveness of long-rest classes. But that's assuming these are combat specialized PCs who have little capability for doing anything outside of combat.
Even though there's only a handful of sub-classes that limited, people do play them, it's rare to see an all-primary caster table. Then there's the Warlock, which, in spite of it's casting focus, is short-rest recharge.
I just don't think I have the energy to throw a half-dozen fights against readily defeatable foes for no purpose other than to drain their resources.
That style is evocative of the classic game, though, and it is meant to (and does) work in 5e.
As long as the two short rests a day are met, the short rest classes balance against the long rest classes.
So, 6-8 encounters/day with a short rest after every second encounter or 3-4 tougher encounters/day with a short rest between encounters, is roughly equivalent? Sounds reasonable to me.
Balancing classes around 6-8 encounters/day was a terrible design decision, a mistake I'm surprised they made after such a long playtest. And yes, we're talking about combat encounters specifically, because the other two pillars drain very few in-game resources.
It's only a mistake if there was a better solution that accomplished all the same things. Remember, 5e was trying to capture classic-game feel. Balance wasn't a priority, classes differentiated by different resources mixes was. If you're going to have long-rest recharge and short-rest recharge and mostly-at-will classes skipping about, feeling neatly differentiated, yet balance them, you're going to need to balance them around some point.
I'd argue that 5e doesn't even really try to balance the classes around 6-8 encounters, it just recommends that as the point where the DM might find it easier to impose balance, if he wants to (not every group needs balanced PCs).
Very helpful, indeed!
So it's pretty much a disaster all around. The vast majority of groups average way fewer than 7 encounters/day, for reasons that are blindingly obvious. I really have no idea why the game is balanced around 7 encounters/day with 2 short rests, rather than (perhaps) 3-4 encounters/day with 1 short rest.
In theory, as Xaviat pointed out, above, it should also theoretically balance with 3-4 harder encounter with a short rest between each of them (2-3 short rests).
Bringing down the number of encounters/day any further would mean needing to reduce spell slots, which would start to lose some of the 'classic feel' that's (IMHO) contributed so much to 5e's success.