It's hardly surprising that your customers who have already become
accustomed to the way your product behaves are annoyed when you change
it. LISTSERV and ListProc users who were used to Reply-To being
set by those servers also tended to think that having lists set
Reply-To was "the right thing".
None of this changes the fact that having lists set Reply-To is less
functional and does harm to the message. All it means is that users
often choose the behavior they have adapted to in their isolated
communities, rather than the behavior that works best overall. More
generally, people tend to keep doing things the same way that they first
learned to do them, even if they find out later that there are better
ways to do that. That's not an argument for propagating bad practice to
new users.
Keith