By the by, where I think WotC really needs to focus next is turning out top tier adventures because most have serious issues. I had plan to run Vecna but all I hear it bad things.
I think that high level adventures are very challenging because high level characters (with their skills, relics and magics) are very challenging.
First, what do people really want and expect from a 16+ level campaign? It's not going to be low-magic. The game isn't magic-less barbarians riding and hunting titanic Beasts for meat and glory. (Well, it can, and that sounds cool... but...) But it uses the assumed multiverse of D&D. You can't be level 16+ and not be famous and powerful. You've done things to earn that experience. BIG things. Fought and Used Big Magic things. You've saved or destroyed cities, if not realms, by now. Is that the kind of story you want? If you don't want that, you might be writing your own adventures anyway that work better for your style.
Second, the characters are so varied in different realms of focus or effectiveness that it's hard to design around. There's pretty fun character concepts that aren't as effective, vs. the extremely effective characters, like, Damage classes pouring Decanters of Endless Damage upon their foes, and Tank/Healer Duos that are nearly indestructible, and many Spellcasters are good at seeing what's coming and influencing the outcome with their knowledge and magic. Characters get such specialized and/or amazing abilities, they can either be useless to a situation, or bypass threats entirely (unless there is a built-in reason to regularly foil the players' tactics, which shouldn't be a crutch). It has to be written in a way that makes sense for such a diverse variety of characters. Our players are mathematical and/or creative, out-of-the-box, geniuses. It's hard to plan for.
Third, the adventure needs a good hook, worthy of Legendary 16th level characters, that has to mean something. Is it just that it defines an era, or becomes the history of the land? Does it have cosmic impact? Can it just be a popular theme like "Space Pirates?" Is it "The Great Red Wyrm vs. Heroes who have collected the Relics of Primordial Power"? It needs something unique that makes it stand out. Is story good enough? Or does a new mechanical expression, or rule, or interesting story structure sound really tempting? Something that might draw attention of, or be of use to, DMs to who use it for their own purposes and inspirations? It needs to be useful. Valuable. Those are the memorable ones. I LOVED Curse of Strahd's use of the Tarokka and randomizing the relics. It made that adventure so much more re-usable.
Sometimes you swing for the fences and miss. But most of the time they don't. And I respect the effort. I already plan on cannibalizing all kinds of ideas, maps, creatures, and encounters, if the adventure doesn't fit my currently planned multi-year campaign, you know?