Comments are absolutely temporary; we are very lucky that the good ones stick around as some of them can be incredibly valuable to future readers, but impermanence is as true in Stack Exchange as (many believe) it is in the rest of the universe.
The doctrine asserts that all of conditioned existence, without exception, is "transient, evanescent, inconstant". All temporal things, whether material or mental, are compounded objects in a continuous change of condition, subject to decline and destruction.
Levity aside, I can recommend that you could consider copying the text of comments that you find helpful and paste them somewhere on your own internet device like a text file.
I once questioned the wisdom of deleting comments in a comment, only to then see an additional swath of comments disappear before my eyes in response†. This was "the universe's" way of telling me about the "impermanent" nature of comments.
Comments being essentially temporary is just how Stack Exchange works, and one of the many reasons I include "let Stack Exchange be Stack Exchange" in my profile as a reminder.
Going with the flow and following the rules and guidelines will pay off better than looking for exceptions.
†I'm quite sure no particularly useful comments were harmed in the making of this tutorial ;-)