It seems to be widely believed that...
"comments are ephemeral" - Everyone on SE
and ephemeral means...
lasting a very short time; short-lived; transitory
So, the facts...
- Comments can only be deleted by the owner, and the select few with Moderator privileges.
- Unlike posts, comments can't be changed/edited (except for the first 5 minutes, and only by the owner)
- There are only 2 reasons to even flag a comment
- Is rude/abusive (applies to everything)
- Is not/is no longer necessary (If comments are really ephemeral, shouldn't these clean themselves up by the definition of ephemeral?)
- Users can't lose reputation because of their comments. (They can be banned, but for the same reasons as all other posts, but harder for Mods to see a behavioral pattern)
The best explanation of this mindset I could find is here
From that answer
The user who posted them or a moderator can delete them at any time, that's just stating a fact, I'm not discussing intent,
So just like posts, only much more restricted. Except for an edge case with questions related to holding you accountable for the people's time you've wasted
Regular users have no way of seeing deleted comments, they are only available to moderators, in contrast with deleted questions and answers which can still be accessed by regular users with more than 10,000 reputation,
cus, ya know, so many people have 10k+ rep
Moderators don't have access to a central list of deleted comments, if they don't know that a comment was deleted they might never find out.
So, because Mods have an incomplete toolset
The reality seems to be that comments are nearly permanent. And carry far less accountability than normal Q&A posts.
It would be interesting to see the average lifespan of a deleted comment vs a deleted post...
We all know SE won't put a Time To Live of comments, because sometimes they are important, and need to live for the life of the Q or A. (Which, is probably a sign that a suggested edit is required, or is in some way an addendum or sub-answer)
So, to reiterate the base question; If comments are ephemeral, why don't they expire? If they are pseudo-permanent, why does everyone say they are ephemeral? In what way are they "short lived" compared to Q&A?