The problem is we don't have the users from 20-30 years ago either.

Now, even though I and my email client still use threading, much of my incoming corpus does not. Instead, I see a mixture of users who break every thread (no In-Reply-To headers, ever), users who cannot comprehend subject metadata versus first line of message, and users who use the most recent few screens of activity as an address book, always replying to the end of a thread with a completely unrelated and new topic.