Helldritch
Hero
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.
Again the famous "basic rule" intended. The more options you add, the more work you'll have to put in your game. But putting advanced challenge making in the hands of the DM was the only choice possible.
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?
With simple tools you can go very far. I agree with you that it is our role to modify the tools given for our tastes and preferences which might not be the same as another DM.
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.
Again, we get to the DM's role. You want to modify the rules? Go ahead but you will have some work to do.
Should they have given more guidelines for "advanced" groups? Maybe. But the variables are way too many to encompass every possibilities. I allow feats but no backgrounds. I allow positioning with flanking, inspirations and hero points but no feats. I do not allow human variant but I allow feats but no options from the DMG. I do not allow demi-humans but I allow feats and hero points.
You get the point. There are so many possibilities that taking all of them into account is impossible. A DM must do some work if he wants to play something more than the base game.