Berthold on his blog wrote a piece about the admonition that a writer should write as simply as possible. My response could not be condensed into a simple reply. I've had to think about it.
One of the greatest tools I used as a 7th-10th grade English teacher was this book. If You Can Talk, You Can Write. I was dealing with students reluctant to write their spelling words into a sentence. This book was a way to get students writing and overcome a reluctance to put pencil or pen to paper. It's baby food. It's a start and hopefully from there they will become capable of writing on their own. It's writing in it's simplest form. It should not be the norm!
Does that mean we should all still want to write like Shakespeare or Dickens? That would be silly and today's audience whose eyes and ears have been captivated by TV and movies wouldn't read it. This was a can of worms to me. How complex is too complex and how simple is too simple?
I equate the keep it simple tripe to "Dumb it down."
I think the wrong word here is simple. My definition would be to make your writing understandable.
A lot of people bought Stephen Hawking's A Brief History of Time, but wouldn't or couldn't understand one word out of ten of what he's saying. It was a way to impress guests at a party by having it on table or bookcase. Isaac Asimov, on the other hand, was a brilliant scientist, but he was gifted with the ability to take the most complex scientific and technological concepts and make them understandable. In Beginnings, he took life from the modern age to the big bang and made most of it is understandable. That is what we should strive for as writers.
If you strive for simplicity you face the horror I did as a teacher with textbook adoptions. History texts expressly. The problem here is what they called readability. If students are only reading at 3rd grade level then a high school textbook should be written at the third grade level. Pure bullshit. You could not believe the garbage they tried to pass off for developmental students. No wonder they can't read, what they're offering is unreadable. Think Donald Trump trying to explain the Declaration of Independence.
When I first taught 7th grade English I found girls were reading Judy Blume, S.E. Hinton, Daniel Steel. Boys were looking at the pictures in Playboy and Penthouse. Not in my classroom, but you get the drift, If they had a National Geographic it was for naked Pygmies.
Growing up, in 7th grade we read Edgar Allen Poe, and I loved his stories. I got a classroom set of Poe's books and had to read it aloud and explain every other word. I tried Mickey Spillane's murder mysteries. The boys yawned. Then I had them read Barry Sadler's Casca books. They are simple, straight to the point, but the history was sound and they could identify with a soldier going through all the major battles of the world since the first century ACE. I found books for boys in their formative reading years that they were interested in and wanted to read. The vocabulary wasn't difficult, but it wasn't dumbed down. It was understandable.