All of this discussion comes from a bad reading of the rules, honestly. The game never advocates a high number of encounters per day.
So no, there is no recommendation, just a simple statement of what COULD be handled IF you only threw medium to hard encounters are a really typical group (which no one ever does anyway). It would be nice if people who are bent on criticizing the system (yes, it's not as precise as 4e because the overall system is more fuzzy, but also way more open-ended, as D&D should always have been) would actually read the rules and stop repeating hearsay that has no ground in the rules themselves...
This comes from the math of the system, specifically the class recovery mechanisms, not just the 6-8 encounters per day recommendation in the DMG.
There are two aspects to number of encounters per day. Of of them is tension and risk, and that can be balanced by making encounters more deadly. However, the other one is attrition of resources, and that one needs a number of actions - usually broken out into a good number of encounters - to balance out.
It has to do with the math behind balancing at-will primary classes (like the rogue or the EB warlock) vs. long-rest-recovery classes (like pure casters outside the warlock) and the hybrids (like barbarian or paladin who need their logn-rest to really make the most of their at-will).
Let's take the simplest example - what's more powerful, a barbarian that can rage every combat or one that can rage half the combats. But it's on classes like the casters it's the most obvious. It really comes down to average effect per action over the course of the adventuring day.
It's a safe statement to say that using one of your highest levgel spells has more effect per action than an at-will character doing a single action. It's also usually safe to say a cantrip action will do less than an at-will character doing a single action. So the place where the casters and at-will characters balance out is when casters are using both high level slots, and enough cantrip actions to balance them down to what the at-will characters will do. This doesn't happen if there as so few combat actions in a day that the casters still have slots left. Heck, it doesn't even happen if the casters run out of slots but just barely. It happens when the casters run out of slots and do a lot of cantrip actions. That takes a lot of actions - a lot of turns of combat.
It's made even harder when condensing to fewer combats that spell with durations can do even more in a longer combat. Shortest durations out there are usually 1 minute. Spiritual Weapon ina 3 round combat will do less than one in an 8 round combat, but it's still the same slot to cast. So when you combine to fewer combats you need to run to even more total rounds than if they were separate combats to have enough low-efficiency actions to balance it out.
And all of that doesn't include short-rests recovery, which is a separate axis as well but with similar effects - a monk will be a lot less effective in a single large combat than in multiple combats and the recommended (and balanced around) 2 short rests around 1/3 and 2/3 of the way through.