I don't necessarily think this has anything to do with mails per se but with the way people use them, which nowadays is just top post all the things

This is not a problem inherent to mail though. If you look at some thread on Lemmy or reddit, you essentially see the same problem. A user posts a long text or comment and makes four, five points that would warrant addressing further. Ideally, you would craft four, five answers and post them as four, five replies, thus giving the discussion a nice structure. What happens instead is that people craft one long reply and keep the mud balling rolling.

Good communication is almost never a question of technology I'd argue.