Thou shalt not dumb down thy writing, but don't make it a vain exercise of style

I'm a non-native English speaker, and let me put it straight: I may write in simple English, because limited are my English writing skills, but I don't want to read simple English because I want to enrich my vocabulary and grammatical constructions.

But whether you write for a native English speaker or not, write clearly, avoiding unnecessary verbosity just to show off your eloquence.

(And, honestly, stop wasting time using those readability tests)

My advice would be to be more direct. Many business and academic documents benefit from more meaty, direct, gutty writing.

You might even improve your own style, for English readers, if you change your attitude. Read the following advice:

https://ocw.mit.edu/courses/media-arts-and-sciences/mas-111-introduction-to-doing-research-in-media-arts-and-sciences-spring-2011/readings/MITMAS_111S11_read_ses5.pdf

In particular see the comments on page 5 about "English teacher beaming at you" and "emphasizing clarity and easy readability". Some of your comments in your question ('dumbing down', 'university students write university level') seem to me to show that you are too in love with showing off. Real good writing is much more about good ideas and good structure and clarity than it is about fanciness.

I think what it comes down to is this: Why do you write and who do you write for? If your a novelist you have a different target audience than if you're a technical writer. If you're a novelist writing romance novels you have a different audience than if you're shooting for a Nobel Prize in Literature. Likewise, if you're a technical writer, your style should similarly be different depending on who your audience is. In all of this, I don't think it's about "dumbing down" your writing as you suggest in the title of your question, but it's a careful consideration of what you are trying to do: namely, to communicate something to a target audience. In your case, it's likely to communicate knowledge, not your intellectual prowess.

So do an assessment: Who do you write for? What do they want to get out of reading what you write? What do you want them to get out of it? And then assess what the answers to these questions mean for how you should write.

This may feel sad: If you have a large vocabulary and are fond of complicated grammatical constructions, you may not be able to use those in your technical writing. (Though, of course, you may get to do that when writing to friends or for other outlets!) But using simple(r) language does not make you a worse writer: Rather, if you manage to adjust your writing style to your target audience, then that's exactly what makes you a good writer!

As hinted at by other comments and answers, the very wording of the premise suggests attitudinal problems...

Clear communication is always the goal. Scholarly writing is not necessarily suppose to be "purely decorative" (that is, non-functional) ... of course depending on one's assumptions about the larger goals.

If a thing is simple, its explanation should be simple. If your claim is that any competent professional should be able to understand it, the writing should accomplish that.

My experience of academic writing, teaching on English for Academic Purposes courses and, lastly, seeing the results of native-speaker academics trying to 'dumb down' their language, suggests that most native speakers have an incredibly poor understanding of what makes a piece of writing difficult for a non-native speaker or reader to understand.

Your peers are used to reading papers in English and, no doubt, have read more succinctly put, more elegant, more descriptively adept pieces than you are going to produce—even though yours will be succinctly put, elegant and descriptively informative.

If you aim for the highest possible standard of academic writing, whatever that may be for your field, then your writing is more likely to coincide with the style, register, tone and range of vocabulary and grammatical constructions that your peers are already familiar with. There is no reason to depart from this. Indeed if you do, you are likely to cause your readers problems. And, of course, you are more likely to distract yourself from the task of expressing ideas in the most natural and effective way you can.

Food for thought, to help you decide for yourself.

Style and substance go hand in hand. In a creative piece, style is more important and calls for that certain flair vocabulary helps us to achieve. On the other hand in a scientific journal, clarity is key. Where does your writing lie in that spectrum?

The point of communicating is to connect with your audience. Don't look at it as dumbing down your language; look to it as using 'appropriate' language.

The intention behind the writing is pretty important too. For example, if you are writing a literature thesis for university it would be reasonable to expect your reader to have an advanced grasp of the language.

Finally, this doesn't need to be viewed as an either or, there may be ways to communicate complexity through simplicity.

My personal opinion: complex vocabulary is overrated if the message can be achieved with simplicity- including in creative pieces. As the Bard once said: Brevity is the soul of wit.

Impress, but don't show-off

Your audience wants to be impressed, but they don't want their time wasted (this is true whether you're writing to an admissions committee, a technical journal, or a picture book aimed at 6-year-olds). I've written and suffered for (and reviewed and condemned for) trying to sound important by using words where I needn't or expressing ideas in the most grand way possible. Oddly, after reviewing/writing somewhat for academia, it becomes second nature to do so, despite it not being the better choice.

The difference between well-written and showing-off is frankly reflected in a ditty by Dr. Seuss:

It has often been said

there’s so much to be read,

you never can cram

all those words in your head.

So the writer who breeds

more words than he needs

is making a chore

for the reader who reads.

That's why my belief is

the briefer the brief is,

the greater the sigh

of the reader's relief is.

And that's why your books

have such power and strength.

You publish with shorth!

(Shorth is better than length.)

Keep it clear. Keep it tight.