It certainly can happen that something reasonable gets a lot of downvotes. But not nearly as often as it happens that something unreasonable (or stupid or irrelevant or otherwise unhelpful) gets a lot of downvotes even though its author thinks it's a useful contribution.
I think "I know I'll get downvoted ..." usually just indicates (not a long tragic history of getting downvoted for insightful comments, but) roughly what it says: the author expects to get downvotes -- and probably hopes to get fewer by saying s/he expects them. HN is better off without those preambles because (1) they're a waste of space -- learning that the author of something expected downvotes tells you nothing useful -- and (2) they distort the scoring system, which (for all its flaws) does help to order comments well and identify ones that aren't going to be worth reading.
Also, the only reason the author would expect their comment to be downvoted is because 1) its insightful, but unpopular and 2) because the author knows right well that the post adds little value, is off-topic or detracts from the conversation.
If its #1, then you must clearly state your argument. A well thought out, but unpopular, opinion normally doesn't get as many downvotes as a not so well thought out opinion.
If its #2, then maybe the author should simply refrain from posting.
I think "I know I'll get downvoted ..." usually just indicates (not a long tragic history of getting downvoted for insightful comments, but) roughly what it says: the author expects to get downvotes -- and probably hopes to get fewer by saying s/he expects them. HN is better off without those preambles because (1) they're a waste of space -- learning that the author of something expected downvotes tells you nothing useful -- and (2) they distort the scoring system, which (for all its flaws) does help to order comments well and identify ones that aren't going to be worth reading.