>The only MUA where reply is complicated is Outlook because it is still not possible to quote easily.
No, there’s certainly complications regardless of client. For example, verifying the reply email is actually from the user, handling top posting vs inline replies, etc. Lots of these systems work around issues by adding things like “REPLY BELOW THIS LINE” and instead of CCing multiple users, sending individual emails with their own personal reply-to addresses. All in all, I’d be happy to see that mess disappear, for things other than mailing lists where the actual entity is an email in the first place.
>Plaintext mail should not just continue to function. A plain text mail should be default.
Well, I just do not agree with that.