> I am deliberately replying in this thread because I often do not find
> it helpful if the list structure of a thread is suddenly turned into
> that of a tree because people start new threads on subtopics. Maybe
> it's just who thinks so.

I think almost everyone finds email trees suboptimal. However, having
all replies to a thread as a flat list also presents its own problems -
it becomes hard to split off sub-discussions.

The Discourse forum software uses an interesting hybrid where each
thread's replies are a flat list, but posts that reply to an earlier
post than the previous one contain a hyperlink to their parent.

Ideally it would be a user interface feature so everyone can choose
their favorite display style for themselves. But not much innovation is
done in email clients anymore. We should start a Scheme messaging client
covering email and IRC :)