I think it would be doing a disservice to learners to not use the words we feel most precisely communicate what we are trying to say. I do make some accommodations when I post an answer, like simplifying my sentence structures and linking definitions for idioms or unusual words, but I try not to "dumb down" what I write.
I have been a voracious reader for as long as I can remember, and if I had limited myself to reading material that consisted entirely of words and that I was familiar with and simple, unchallenging grammar, I would not have the vocabulary nor the writing ability that I do. I think there is value in writing an answer using the words and phrases that we would normally use with some minor adjustments for our audience.
If I were speaking to someone just learning English, I would slow down, enunciate, shorten my sentences and avoid slang and unusual words. Writing is different. There is time for a learner to pick apart what we've written and look up words and to digest the meaning. If something is unclear, a learner can take the time to write a comment to ask for help without worrying about their pronunciation or feeling pressured.
The "Drawing Accuracy, Quality and Expertise" sample reads to me as if a middle school student wrote it. I found that the lack of precision and detail made it difficult to understand. A child would start a school term paper with a statement like "I study drawing because it’s a good way to understand how people draw". That is not the style that I think most learners would aspire to, and I would be embarrassed to post something written that way. It's not my voice - I would rather my writing be a bit too challenging than for it to be patronizing.
If someone else would like to write in "up-goer five" style, I think that's great. The more diversity there is in the style and tone of answers, the better. I personally think that some people learn best by immersion and that replacing an uncommon word that means precisely what we're trying to say with half a sentence of common words that vaguely resemble what we're trying to say and that we wouldn't normally string together doesn't help their English improve.
I also think that limiting the vocabulary isn't the best way to make something easy to read. There's no guarantee that the words that English speakers most commonly use are the same vocabulary that is being taught to learners. There are some words that aren't common, but when you need to express an idea, there's no easily understandable replacement. For example "lower students" and "low students" is not a suitable replacement for beginner. The students aren't lower; they have a lower level of skill.
Simple sentence structures, good formatting and punctuation can go a lot further than limited vocabulary. I think a better test of readability would be to paste your answer into a translator, translate it to another language, then translate that back to English and see how mangled it gets. Translating one of the "up-goer five" paragraphs in your question to Spanish and back to English results in:
I was a little sad, I must say, in the way we responded and also on the answers given. Although I agree that we need all kinds of answers here, it's definitely a good thing to make our writing easier for the greatest possible number of people to read. Must have at least some answers that younger students can read and understand without having to go find a lot of words up.
The simple sentence structure did more to preserve the meaning than the arbitrary substitution of "lower" for beginner. If a student wasn't familiar with "lower student" and went to look it up, they would think you're talking about younger students, not beginners. I think that it is even more important to be precise when you're writing for someone that may have to resort to a dictionary to understand.