[leni536]

> The only issue is that common mail clients still automatically quote the whole message being replied to for no good reason.

Here is a good reason: In-Reply-To is a reference, not content. The recipient(s) of your message might not have that email.

Also including the quote is a default. The sender can edit it, splice responses into it and remove irrelevant parts of it. Admittedly quoting norms are in shambles though for various reasons.