The MUA has a choice. Either is can be conservative in what it
generates and liberal in what it accepts, or it can blindly generate
whatever it accepts, or it can make a smart judgment.
To me, being conservative in what it generates means not decoding and
reencoding things it doesn't understand from a message being
replied-to - it means keeping things as they are.
I agree. But what "are" things when a MUA sends a reply? IMO, things
"are" whatever the user sees on-sceen when issuing the "send" command,
and that is what MUA should keep.
I see your point. I suppose I would say that if the MUA does reliably
understand the charsets in the original message, it should use the same
encoding for the reply if the subject isn't changed. But the receiving
MUA is having to use heuristics to guess what the encoding is, maybe
it's reasonable to re-encode the result to say "this is what the
responder thought the subject was".