Please note this moderator feedback from a flag I raised a while back:
moderators do not determine the validity, accuracy, or quality of answers
Incomplete, speculative, superficial, redundant, and incorrect answers are still answers. StackExchange offers several ways to offer feedback for such answers and improve them: don't accept, leave comments, edit, down-vote. However, the “not an answer” flag is isn't a good way to deal with low-quality answers. Emphasis mine:
This was posted as an answer, but it does not attempt to answer the question. It should possibly be an edit, a comment, another question, or deleted altogether.
This flag is not for reporting poor attempts at an answer. It's for reporting cases where somebody has mistakenly posted a followup question or comment in the answer box. When you post that a good-faith answer should have been a comment instead, it's actually a bit insulting, as it suggests that the poster has not even attempted to give an answer and has possibly made a newbie mistake.
Some StackExchange sites (like Skeptics) ban partial or unsourced answers, but English Language & Usage is not one of them. Etymology in particular is often unknown or obscured by folk etymology, and in those cases we do our best to present the available evidence, often by citing the earliest known usage in print. It's unfortunate that we can't always provide a definitive answer, but calling people out for posting “comments” as answers is non-constructive and rude.
In a few cases, you have actually pressed people to dig up better evidence, which is awesome. However, the way you've gone about that has struck me as abrasive, downright rude sometimes. Please be more sensitive about eliciting better answers, rather than chiding people for posting “long comments” as answers and similar tactics.