This is all a matter of the email client. Useful email clients show email threads in trees like this (the red lines and arrows): https://vigasdeep.com/2012/06/07/mutt-the-ultimate-mailing-c...
"Just use mutt" as a reply to "I want to read this one email thread" is rather missing the point. Plus the reality is most people neither use nor want mutt. Many people think the UX on mutt is horrible. Nice if it works for you, but it doesn't for many. So there's that.
https://lwn.net/Articles/875239/ https://blog.gnoack.org/post/lei/ https://people.kernel.org/monsieuricon/lore-lei-part-1-getti... https://b4.docs.kernel.org/ https://github.com/mikwielgus/forum-dl
If you’re downloading mbox files, then you know how to handle them.