Your experience is the same as mine in regards to merge proposals, and it's why I always try to update my merge proposal comment after the merge is completed. I also try to put links to the profiles I'm talking about, but that doesn't also help when profiles are merged. (I was working on a profile recently & it took me entirely too long that I was trying to research an issue brought up in a comment that was about an entirely different person - the never-left-England John who had been incorrectly merged with the died-in-Virginia John. Very frustrating.)
Anyway. For now, for Alice, I'm waiting to hear from the person who uploaded the image. Since we have no idea where it came from or who it is (except not Alice, by what she's wearing), I think that it will probably be removed from WikiTree, in which case I'll see if I can delete the comments. Might be that I'll only be able to archive them. Haven't quite figured out who can do what since the new comment system was implemented.
Personally, I like to see the comments, regardless of their status. At the least, they speak to the collaborative efforts on the profile and they record any deliberation about something. I must admit that I get a bit annoyed when someone deletes a comment I posted. There's generally no way for me to know what that comment had been about (and yes, I've had important/difficult to replace comments deleted by PMs who did not want to hear anything that might show a flaw in their preferred lineage - just takes one time for that to happen to realize why deleting comments shouldn't be made easy).
P.S. I've been known to do "slash and burn" edits, but generally only upon request (when I was working as an editor). I was also a writer who was edited, so I know how it feels. At one publication, the editor used a green pen & we used to say we'd been slimed... it was about the same time that Bloom County, IIRC, has an "I've been slimed" storyline. I asked her when I left the pub why she used green. She said she felt it wasn't as harsh as red. I used a regular pencil when I was editing... my auditors used to complain of lead poisoning :D