What people really want is to type their reply without having to include the original message in their response. That's how most people comment on places like Hacker News and Reddit.
Who said they have to? I doubt many people even care, previous messages are just included recursively by all the major email client apps by default. I suspect most of the people don't even know.
The interesting thing is that if you use a conventional email client that supports actual threading using the Message-Id and In-Reply-To and References header values, and you edit the email message to trim quoted content and reply inline, the conversation view implementation in their email client breaks and it shows a blank message.