6-8 may not be necessary, but multiple encounter days certainly are. They're also strongly supported by genre fiction. Tons and tons of examples...take out the guards, then sneak into the base in disguise, take out some more guards, try to sneak into the detention block to free the princess, get discovered and into a firefight, escape the fight by jumping into a chute, discover that the chute leads to a pit that contains a monster, then discover that the pit is actually a trap as the walls start closing in, then after disarming the trap encounter some more bad guys, then the party gets split (!), one group has to swing actoss a chasm on a rope before finally making it back to the ship where they then watch a climactic battle between the friendly old mentor and the evil overlord....and then fight some more bad guys before finally escaping.
Is it 6 to 8? I don't know...there's some blending in there probably. But it's a perfect example of a multiple encounter day. And to counter arguments...no, not everyday needs to be like this. And yes, there are plenty of examples of days with fewer encounters in movoes and other fiction. But this multiple encounter day (or at the very least the possibility of it) is assumed by the game design and is just as supported by fiction as anything, especially within the history of this specific medium.
As for the design assumptions, I would agree that perhaps more guidance on how to adjust the CR/XP systems to allow for feats and multiclassing was in order. I think it was actually smart to design based on the assumption those options would not be used because of the way they decided to present the game, abd the fact that they are targeting new players as well as trying to retain or regain old players.
Anyone can play the game with the Basic Rules, which are free. The Basic Rules gives one feat and one subclass for each class, so there is incentive to actually buy the books if a new player decides they like the game. So an approach designed to add these kinds of things on as you go just makes sense to me. Especially compared to an approach that assumes the inclusion of all options, requiring peole to remove the ones they don't like and adjust accordingly.
This puts the modification in the hands of players and DMs who have decided to make their game more complex, rather than requiring modifications by people who want a simpler game.
Sure, some more advice on how to adjust may have been in order...but I also think that there is such a broad spectrum of how the game can go that it's hard to know how to make such adjustments. Maybe it is better to leave that up to the individual groups to decide. I mean, if I allow Feats in my game, and my players take things like Actor and Linguist because they think it will be cool for story purposes...does my game require as much combat adjustment as someone who's got players taking Great Weapon Master and Sharpshooter, and also has multiple PCs dipping into Warlock for Devil's Sight so they can cheese Darkness and destroy their foes? Probably not.
So although more advice would help, I think that ultimately they made the right choice. Everyone's game is different and you know it better than anyone. And the best way to learn is through experience; the best advice in a book can only ever hope to match actual experience.
So, to come back to viability...the answer seems obvious to me. I've been given a set of basic tools, and then many examples of how to adjust those tools for a specific purpose. I can indeed achieve a viable high level game using what the books offer. That does not mean I have not had to put some effort in. But when does a DM not have to put in some effort?
So the question seems to me to he about where the designers' responsibility for how the game plays ends and where the DM's begins. It's an interesting question....and with a myriad of answers, I'm sure. But to me, it's about the level of fun I'm having and that my players are having. If the rules are so bad that the game is not enjoyable, then that's the designers' fault. If the game is enjoyable, but there are some areas that can be improved...a bit of a gray area, but hard to say designers of something enjoyable have failed.
But, if the game is mostly enjoyable, and the designers have offered examples of variant rules and options that can help the game be more to your liking, and you choose nit to inprove your game, ,I do think that's a case where it falls to the DM.