What do you do when they don't understand?
In science, an academic paper leaves the understanding to the reader. The author of the paper plows ahead and assumes that the motivated reader will do the necessary work to catch up, fill in and understand what's going on.
In some popular magazines, when the going gets tough, the writer glosses over the difficult parts. She dumbs it down or leaves out the bits the editors assume will confuse readers.
And what about the CFO, writing a memo? Or the engineer, writing out the instructions?
Many sources, from textbooks to websites, take the position that if you don't understand a concept or a nuance, it's your loss. I think that's an strategic failure on the part of the writer. (I'll give scientists and other professional writers a pass.)
Just recently (a decade or so) we opened two doors that change the way we communicate: we can link now, which means that any time you're worried you've hit something too complex, you can easily link to more data and more explanation, and second, you can keep writing. Length (given appropriate organization) is no longer an issue.
At the same time, there's an onus on the reader to look up words and references that are easily found in a search engine before giving up.
Ikea, then, should quit trying to jam nonsense instructions with no words on tiny sheets of paper and should instead post videos or detailed instructions in native languages online. Annual reports should get significantly longer (with better hyperlinked indexes), not shorter.
No one is going to read the whole thing, ever again. But we need to make it much easier to read the part of the thing that someone really cares about.