Q, I did not say that I did not like or value the works of Dorothy L Sayers, which I do, but I do feel there was a fault in assuming the knowledge of a language taught to few in the country at the time. It can make the reader feel that their ignorance (for which they bear no blame) is seen as something of a fault in them. There is a trace of that in your comments, is there not? Classics was, and is, a very small part of what happens in education. Its place in the modern world could be argued. I did science.
Oddly, I do not find Victorian writing or Edwardian writing difficult in the same way, in fact, revel in the complex sentences and syntax. I thnk that where the language is used as the writer used it in everyday life, it is much more comprehensible than where it is not - and I doubt if Donaldson mixed with people who talked like his books.
As for Potter, I so remember that "the effect of lettuce is soporific" (no longer, due to modern breeding). But the meaning was obvious in the story.
I was very interested to discover several pages devoted to Donaldson's vocabulary. I was not the only one with lists and extra dictionaries... And I should point out that if the work was not seen as worth the effort, there would have been none of that at all. And no further purchases of his books.
The worst example I have ever seen of the use of words to obfuscate an issue was a thesis on the aetiology of illiteracy. Any random page did not look like English, but had a pattern resembling Latin. It used so many Latinate and Hellenic words that there were no little linking words, no prepositions or what we had to teach as connectives, as they were all contained within the polysyllabic terms. It started right from the abstract and page 1, no explanation of terms, and the meaning impossible to extract without that large dictionary. I don't know how the friend who showed it to me, who was supposed to be referring to it for a Diploma in Education, managed to do so. I don't know how the author managed to write it. I can only assume it to have been a way of fitting his ideas (I assume he, maybe wrong about that) into the strict word limit. I learned the word aetiology from it. Otherwise, zilch. It did raise the question of where illiteracy lies when the written word somehow fails to communicate anything of value. Obviously, neither Sayers nor Donaldson can be faulted in this way.
You used the word "arcane". That is defined as "requiring secret knowledge to be understood; mysterious; esoteric", or "known or understood by only a few". Precisely my complaint about it. Language is supposed to communicate, not exclude.

Penny