This may be good advice for an instruction manual or children's books, but I think good prose is something of an art in and of itself. Some of the most successful writers do the opposite. Look at the articles that go viral ..you see that the prose is varied and even complicated, not just simple. The best prose is that which conveys the intended message for the intended audience in the best way possible. I think it's…
A memorable critique, from an English professor, of my writing: "stop trying to sound smart". He meant that I should stop trying to use big words and stop trying to construct complex sentences.
>People state they want/like simple writing but revealed preferences show otherwise.
Simplicity isn't what you think it is. The simplicity people want is sentences that flow regardless of complexity. You say complicated, I say clunky. An author might use a complex sentence to coalesce several ideas, but if that complexity, because the construction interrupts the flow, requires the audience to reread the sentence, the author has failed.
That's how we arrive at simplicity. I find it easier to pepper diversity into my prose ex post facto than try during first draft. Connecting two simple sentences is less work than unwinding a complicated sentence.
The advice isn't really to leave your writing simple, it's to start simple. At some point, you will be required to use a complex sentence to connect ideas.