This is a curious paradox. Do the readers with overly simplistic vocabularies really read that much, because one way we increase our vocabulary is by reading. If not, should be dumb down our writing to appeal to people who likely don't read that much anyway? [note this doesn't apply to someone reading in a non-native language.] Most readers have decent heads on their shouders. A huge mistake a writer can make is to under-estimate the intellectual abilities of his/her readers. So what next? Dumb down the plot? Make sure every little twist is underlined and highlighted so people who have a third grade reading aptitude will get them?
As writers we should be chosing the best words for a specific passage, and the common nature of the candidate word should not be at the top of the filter for that selection.
I find it strange that writers should be encouraged to limit the lifeblood of their craft--words.
If a writer is using big words for the sake of using big words, it will usually be obvious to most readers.
A good writer who finds that a best word is an uncommon one should have enough context clues in the surrounding prose that an enlightened reader will be able to get the gist without reaching for the thesaurus. And even if he/she does have to make that reach, gee...so sorry to force you to learn a new word.
[the following example is dedicated to thinker, scarlet, and maryn] If I refer to a slathering vulva, and the reader is unfamiliar with both of those words, he/she should be able to get enough information from the surrounding prose to see that those words do not refer to a well-polished Swedish automobile, but rather to something quite smegmatic.
Monosyllabic isn't. (Many larger words are made up of common roots and pre-/suffixes so the readers can easily figure them out).
A challenging read is not a bad read as long as the challenges flow well with the story.
Writers should always write for the readers, and while this means the writer should not choose big words to make him/her appear to be intellectually cool, it doesn't mean he/she should avoid large words if those words are the best ones for the chosen passage, and the best way to express that part of the story.
Reading is not a total free ride. If the story is a good one, it will require the reader to use his/her brain.