I would think the principle reason for leaving answered and accepted questions open is because it allows users to submit new information, new suggestions, and new answers.
It seems to me that 14 answers is a pretty exhaustive list, the accepted answer itself contains a list of fourteen words! Five out of the fourteen posts have been deleted by mods / the community, which hasn't harmed the question in the slightest. The answers were all very low quality and/or repeating suggestions submitted earlier.
Let's remind ourselves that closed questions are not hidden from view, anyone can look up the question and find that extensive thesaurus list. A closed question just means new answers cannot be submitted.
Is that such a bad thing? Why? If the OP can provide a convincing argument, I'll happily VTR.
Is this question still off topic?
In my opinion, it is still off-topic.
The edit, written by a different user than the OP, states
thesaurus.com doesn't give anything which leads to a suitable noun.
Not true! I clicked on the link and it has the following list
somniferous somnolent soothing soporific soporose trance-inducing torpid
drowsy lethargic listlesss sluggish asleep comatose draggy hypnotic
Included in that list are the words hypnotic, soporific, and drowsy. Amazingly, the accepted answer provided a list of drugs and agents (dwale, somnifacient, somnivolency/somnolency) which induce sleep (or death!), which boggles the mind but the OP never said he was looking for an adjective or a noun to describe sleepy. It seems he was looking for the names of medicines or the agent responsible for making one sleepy. In which case, the question, as it stands, is unclear, and ergo off-topic.