what's broken is something else - perhaps our expectation that all
messages in a thread should have the same subject.
Users often reply to mail in order to start a new thread with the same
set of folks in it. Sometimes they change the subject when they do
this, sometimes they don't. You can bet they NEVER edit the
References or In-Reply-To fields before they send the mail (a few
members of this mailing list excepted, no doubt).
So if you don't consider subject when doing threads, you lose one
thing that users actually DO occasionally use to mark a new topic.
a changed subject is not a reliable indicator of a new thread.
just because the author of a reply changes the subject from that of the
message being replied to doesn't mean that the reply is unrelated to
the message being replied to, and it doesn't mean that recipients won't
want to see the relationships between the messages.
users are lazy - they will sometimes reply to a message just as an easy
way to get the same recipient list, even if the topics are different.
often they don't even change the subject. (but how many MUAs give
users a better way to do this?)
Keith