Parmandur
Book-Friend, he/him
I don't agree that players and customers are particularly satisfied with the results, and the circular logic remains in place. You assert that, because official adventure sales are allegedly good, people must be satisfied. This is does not follow at all. On the contrary, the inability of people to make their own adventures work as well as the frankly weirdly designed WotC ones may well drive sales, as people seek something that works better. With 3.XE and PF there can be not the slightest shred of doubt that the complexity and effort involved in constructing encounters for those systems helped drive adventure and AP sales. Thus I suggest sales of official adventures may well reflect a problem with the system, not satisfaction. I have never seen a WotC survey result saying "Oh yes we love 5-8 encounters per day!" even.
As for "they work fine with less", you're contradicting your own, recent statements! Where you called less than 6-8/day "wonky"! Correctly, I would say. They work poorly with less, but player instincts are to pull back earlier and it's hard to write anything but a dungeon where 5-8 encounters in a day doesn't seem contrived in the extreme (certainly if it keeps happening! One day can work, but two or three or more?).
The guidelines do not explain how to make a "viable challenge" because 5E is badly designed here and utterly reliant on spamming encounters in a ludicrous way for challenge, and ill-suited to typical D&D play. And players do notice. My own players have commented that 5E encounters are a lot easier than 4E, and even if I dial the difficulty up, as you say, that makes things "wonky" because the encounters simply become swingy and deadly and blow resources in a disorderly way.
TLDR - 5-8 encounters per day forces DMs to write around this bizarre requirement, and that's perverse because design should serve DMs, not vice-versa. But if it sells adventures and APs I am indeed sure WotC is richly satisfied.
WotC reports satisfaction numbers with the game as a whole in the 90% range. I see some people on the internet expressing frustration with the adventure day as set up, but rather muted and not offline at all. The books have guidelines on how to brew it at home, and they have published thousands of pages of examples now. It is still an artform and not a science, but it is well supported and embraced by the player base.
Lower encounter days are wonky in terms of things like Wizard or Paladin balance: some Classes get more powerful if they can Nova confident that they won't get in trouble, while other Classes won't get their non-Nova chance to shine if the Nova capable Classes aren't pushed past their limit. But a lot of people don't care about that (again, I point to Critical Role), and have fun anyways. The game is a game of attrition, and not everyone pushes it to the limit. If it was changed to a game that challenged players with a handful of encounters, the attrition game becomes untenable, which wouldn't go over well.