Is it reasonable to get downvotes just because the downvoters' point of view differs from yours, no matter how many valuable references you have provided? Please see this: Where is the subject in "as was traditional for unmarried women"? (Comments included, if possible, as proof of my efforts to reach consensus.)
People may downvote whenever they think an answer is not useful. Sources are good in general, but they aren't guaranteed to be useful. It's possible for sources to be incorrect, and some people might judge an answer that cites incorrect (in their view) sources as "not useful". I wouldn't worry about it too much. Your answer currently has a net positive score, and it was at a disadvantage because the sources were not present initially (didn't most of the downvotes occur before you edited the question to add references?). I don't think it is likely to keep accumulating downvotes.
The main analysis you present, that "as" is a relative pronoun, seems to be disputed by several recent analysts. I understand that you don't fully endorse this analysis, you are just describing it, but I think you may put too much emphasis on Fowler. Fowler was an influential figure for the "prescriptive grammar/style/usage guide" tradition, but as far as I know he is not actually considered a particularly important analyst of grammar from a linguistic perspective. (This isn't intended as a knock on Fowler—I own the new printing of the first edition, but see for example this Language Hat post about David Crystal's introduction to this edition.) Fowler is also not especially recent.
I think your answer might gain more points if you made a new edit that focuses more on Simon-Vandenbergen and Noël's summary of the viewpoints of various more recent grammarians, which seems very helpful.
But you certainly don't have to do this. As I said, I think your answer is fine as it is, and I will be surprised (and disappointed) if its score decreases significantly in the future.
I was somewhat involved with this business, and down votes mean little to me. For me , it is the practice of grammar that is important, not the theory, or, theories. Without proper care, grammar theory can take precedence over grammar practice.
I have had some difficulty adjusting to the protocols of this community. I am trying to adjust, and making some progress. If someone wishes to slap a down vote on me, that is not my business. My business is improving how I practice English, if I can. My only concern with down votes is they often come with no explanation. This practice seems pointless.