I don't believe that my points contradict each other. So I'm going to address that part of your post, to avoid any confusion. But I think the rest of your argument is great, I just want to explain myself a bit better.
For starters, simpler writing can indeed be more unclear (because it's less precise, as discussed). So "simpler, but unclear" isn't contradictory in that sense. In this case the lack of clarity comes from a mismatch in the shared understanding between the writer and the reader. The more "simple" a sentence, the easier it is to misunderstand (to a point, and with exceptions).
And with respect to idioms, I would say that "uses idioms" does not convey the same idea as "idiomatic." By "idioms" I refer to reasoning by a shared analogy. A random example is the idiom, "Like water off a duck's back." It conveys meaning by analogy. This is usually less precise than using a direct description ("He didn't seem to care at all.") Sort of a dumb example, but I hope it makes sense.
Finally, about jargon -- by definition jargon is not part of shared vocabulary outside of a certain field of expertise. I believe a lot of our most precise words, which previously were common usage, are now becoming jargon. This is fine for experts but it leaves non-experts with a paucity of words to express their ideas in novel domains.
(Once again, this is just my personal impressions, not based on studies or anything. So, don't take it too seriously!)
I see where you're coming from, but I think one way in which we diverge is in our ideas about what is valuable in language. I quite like inaccurate, loose formulations because they're playful - and I think writing is a form of play, as much as it's a tool to communicate with. In its place, evocative vagueness can be better than sharp clarity.
My feeling as to what distinguish good writing from bad is more that good writers say what they want. Bad writers say stuff they didn't mean to, say stuff that other people want (cliches), or say nothing at all (obscurity). So in my eyes, there's no distinguishing textual characteristic of good writing. For instance, while I generally agree with you on jargon, David Foster Wallace uses jargon in a way I really like, and think is pretty central to what makes his writing good.
Genre is important here. One's choice of language when playfully communicating with friends will be different than the language used to communicate in a legal brief or a scientific paper. We adapt our language to our audience and the occasion to communicate our ideas. Being thoughtful and considerate to our audience is important on all occasions.
I don't believe that my points contradict each other. So I'm going to address that part of your post, to avoid any confusion. But I think the rest of your argument is great, I just want to explain myself a bit better.
For starters, simpler writing can indeed be more unclear (because it's less precise, as discussed). So "simpler, but unclear" isn't contradictory in that sense. In this case the lack of clarity comes from a mismatch in the shared understanding between the writer and the reader. The more "simple" a sentence, the easier it is to misunderstand (to a point, and with exceptions).
And with respect to idioms, I would say that "uses idioms" does not convey the same idea as "idiomatic." By "idioms" I refer to reasoning by a shared analogy. A random example is the idiom, "Like water off a duck's back." It conveys meaning by analogy. This is usually less precise than using a direct description ("He didn't seem to care at all.") Sort of a dumb example, but I hope it makes sense.
Finally, about jargon -- by definition jargon is not part of shared vocabulary outside of a certain field of expertise. I believe a lot of our most precise words, which previously were common usage, are now becoming jargon. This is fine for experts but it leaves non-experts with a paucity of words to express their ideas in novel domains.
(Once again, this is just my personal impressions, not based on studies or anything. So, don't take it too seriously!)